hantek-6xxx: Ignore requests to set Hantek 6022BE coupling.

Here's a patch to "Ignore requests to set coupling for the Hantek 6022BE",
this clears the LIBUSB errors for me.

Also in the patch:

 - There is a crash because config_list() can be called with sdi == NULL.

   This can be reproduced by doing:
     "sigrok-cli.exe --driver hantek-6xxx --show"

 - There seems to be a very unsafe loop in config_set() when setting COUPLING;
   the coupling vector is assumed to be zero terminated, but is not declared
   as such.

   Note: The same issue is present also for other hardware, at least for
   hantek-dso/api.c. The patch is only for hantek-6xxx though.

src/hardware/hantek-6xxx/api.c

@@ -62,12 +62,12 @@ static const struct hantek_6xxx_profile dev_profiles[] = {
 	{
 		0x04b4, 0x6022, 0x04b5, 0x6022,
 		"Hantek", "6022BE", "hantek-6022be.fw",
-		dc_coupling, ARRAY_SIZE(dc_coupling),
+		dc_coupling, ARRAY_SIZE(dc_coupling), FALSE,
 	},
 	{
 		0x8102, 0x8102, 0x1D50, 0x608E,
 		"Sainsmart", "DDS120", "sainsmart-dds120.fw",
-		acdc_coupling, ARRAY_SIZE(acdc_coupling),
+		acdc_coupling, ARRAY_SIZE(acdc_coupling), TRUE,
 	},
 	ALL_ZERO
 };
@@ -440,13 +440,13 @@ static int config_set(uint32_t key, GVariant *data, const struct sr_dev_inst *sd
 			break;
 		case SR_CONF_COUPLING:
 			tmp_str = g_variant_get_string(data, NULL);
-			for (i = 0; devc->coupling_vals[i]; i++) {
+			for (i = 0; i < devc->coupling_tab_size; i++) {
 				if (!strcmp(tmp_str, devc->coupling_vals[i])) {
 					devc->coupling[ch_idx] = i;
 					break;
 				}
 			}
-			if (devc->coupling_vals[i] == 0)
+			if (i == devc->coupling_tab_size)
 				ret = SR_ERR_ARG;
 			break;
 		default:
@@ -465,7 +465,7 @@ static int config_list(uint32_t key, GVariant **data, const struct sr_dev_inst *
 	GVariantBuilder gvb;
 	unsigned int i;
 	GVariant *gvar;
-	struct dev_context *devc;
+	struct dev_context *devc = NULL;
 
 	if (key == SR_CONF_SCAN_OPTIONS) {
 		*data = g_variant_new_fixed_array(G_VARIANT_TYPE_UINT32,
@@ -477,10 +477,8 @@ static int config_list(uint32_t key, GVariant **data, const struct sr_dev_inst *
 		return SR_OK;
 	}
 
-	if (!sdi)
-		return SR_ERR_ARG;
-
-	devc = sdi->priv;
+	if (sdi)
+		devc = sdi->priv;
 
 	if (!cg) {
 		switch (key) {
@@ -506,6 +504,8 @@ static int config_list(uint32_t key, GVariant **data, const struct sr_dev_inst *
 				devopts_cg, ARRAY_SIZE(devopts_cg), sizeof(uint32_t));
 			break;
 		case SR_CONF_COUPLING:
+			if (!devc)
+				return SR_ERR_NA;
 			*data = g_variant_new_strv(devc->coupling_vals, devc->coupling_tab_size);
 			break;
 		case SR_CONF_VDIV:

src/hardware/hantek-6xxx/protocol.c

@@ -220,9 +220,13 @@ SR_PRIV int hantek_6xxx_update_coupling(const struct sr_dev_inst *sdi)
 	struct dev_context *devc = sdi->priv;
 	uint8_t coupling = 0xFF & ((devc->coupling[1] << 4) | devc->coupling[0]);
 
-	sr_dbg("update coupling 0x%x", coupling);
-
-	return write_control(sdi, COUPLING_REG, coupling);
+	if (devc->has_coupling) {
+		sr_dbg("update coupling 0x%x", coupling);
+		return write_control(sdi, COUPLING_REG, coupling);
+	} else {
+		sr_dbg("coupling not supported");
+		return SR_OK;
+	}
 }
 
 SR_PRIV int hantek_6xxx_update_channels(const struct sr_dev_inst *sdi)

src/hardware/hantek-6xxx/protocol.h

@@ -99,6 +99,7 @@ struct hantek_6xxx_profile {
 	const char *firmware;
 	const char **coupling_vals;
 	uint8_t coupling_tab_size;
+	gboolean has_coupling;
 };
 
 struct dev_context {
@@ -127,6 +128,7 @@ struct dev_context {
 	int coupling[NUM_CHANNELS];
 	const char **coupling_vals;
 	uint8_t coupling_tab_size;
+	gboolean has_coupling;
 	uint64_t samplerate;
 
 	uint64_t limit_msec;