Don't automatically clear known instances from USB drivers on scan

This invalidates previously returned sr_dev_inst pointers, which a
frontend may be holding. It's the frontend's responsibility to clear
the list of instances a driver keeps track of by calling

	sr_dev_clear(driver);

if it wants a completely new scan done.

hardware/asix-sigma/asix-sigma.c

@@ -431,8 +431,6 @@ static GSList *hw_scan(GSList *options)
 
 	devices = NULL;
 
-	clear_instances();
-
 	if (!(devc = g_try_malloc(sizeof(struct dev_context)))) {
 		sr_err("%s: devc malloc failed", __func__);
 		return NULL;

hardware/fx2lafw/fx2lafw.c

@@ -371,9 +371,6 @@ static GSList *hw_scan(GSList *options)
 
 	drvc = di->priv;
 
-	/* This scan always invalidates any previous scans. */
-	clear_instances();
-
 	conn = NULL;
 	for (l = options; l; l = l->next) {
 		src = l->data;

hardware/hantek-dso/api.c

@@ -269,13 +269,10 @@ static GSList *hw_scan(GSList *options)
 	const char *conn;
 
 	drvc = di->priv;
-	drvc->instances = NULL;
 
 	devcnt = 0;
 	devices = 0;
 
-	clear_instances();
-
 	conn = NULL;
 	for (l = options; l; l = l->next) {
 		src = l->data;

hardware/lascar-el-usb/api.c

@@ -85,9 +85,6 @@ static GSList *hw_scan(GSList *options)
 
 	drvc = di->priv;
 
-	/* USB scan is always authoritative. */
-	clear_instances();
-
 	conn = NULL;
 	for (l = options; l; l = l->next) {
 		src = l->data;

hardware/nexus-osciprime/api.c

@@ -157,9 +157,6 @@ static GSList *hw_scan(GSList *options)
 
 	drvc = di->priv;
 
-	/* USB scan is always authoritative. */
-	clear_instances();
-
 	conn = NULL;
 	for (l = options; l; l = l->next) {
 		src = l->data;

hardware/uni-t-dmm/api.c

@@ -86,9 +86,6 @@ static GSList *hw_scan(GSList *options, int dmm)
 
 	drvc = udmms[dmm].di->priv;
 
-	/* USB scan is always authoritative. */
-	clear_instances(dmm);
-
 	conn = NULL;
 	for (l = options; l; l = l->next) {
 		src = l->data;

hardware/victor-dmm/api.c

@@ -95,9 +95,6 @@ static GSList *hw_scan(GSList *options)
 
 	drvc = di->priv;
 
-	/* USB scan is always authoritative. */
-	clear_instances();
-
 	devices = NULL;
 	libusb_get_device_list(drvc->sr_ctx->libusb_ctx, &devlist);
 	for (i = 0; devlist[i]; i++) {

hardware/zeroplus-logic-cube/api.c

@@ -290,8 +290,6 @@ static GSList *hw_scan(GSList *options)
 
 	devices = NULL;
 
-	clear_instances();
-
 	/* Find all ZEROPLUS analyzers and add them to device list. */
 	devcnt = 0;
 	libusb_get_device_list(drvc->sr_ctx->libusb_ctx, &devlist); /* TODO: Errors. */