ftdi-la: silence "assigned but never used" compiler warning

Stick with the prepared but unfinished "limit msec" code path, but
silence the warning about an unused variable value.

This was reported by clang's scan-build.

src/hardware/ftdi-la/api.c

@@ -346,6 +346,7 @@ static int config_set(uint32_t key, GVariant *data,
 	case SR_CONF_LIMIT_MSEC:
 		value = g_variant_get_uint64(data);
 		/* TODO: Implement. */
+		(void)value;
 		return SR_ERR_NA;
 	case SR_CONF_LIMIT_SAMPLES:
 		devc->limit_samples = g_variant_get_uint64(data);