soapy: use SoapySDRDevice_enumerateStrArgs to create Soapy args object

this allows to use Soapy device parameter when creating the device

lib/src/phy/rf/rf_soapy_imp.c

@@ -286,7 +286,7 @@ float rf_soapy_get_rssi(void* h)
 int rf_soapy_open_multi(char* args, void** h, uint32_t num_requested_channels)
 {
   size_t          length;
-  SoapySDRKwargs* soapy_args = SoapySDRDevice_enumerate(NULL, &length);
+  SoapySDRKwargs* soapy_args = SoapySDRDevice_enumerateStrArgs(args, &length);
 
   if (length == 0) {
     printf("No Soapy devices found.\n");