Fix Icom icom_get_usb_echo_off to read the freq response when echo is on

This takes into account whatever the response time of the rig is
https://github.com/Hamlib/Hamlib/issues/615

rigs/icom/icom.c

@@ -730,8 +730,13 @@ int icom_get_usb_echo_off(RIG *rig)
 
     if (ack_len == 1) // then we got an echo of the cmd
     {
+        struct rig_state *rs = &rig->state;
+        unsigned char buf[16];
         priv->serial_USB_echo_off = 0;
         rig_debug(RIG_DEBUG_VERBOSE, "%s: USB echo on detected\n", __func__);
+        // we should have a freq response so we'll read it and don't really care
+        // flushing doesn't always work as it depends on timing
+        read_icom_frame(&rs->rigport,buf,sizeof(buf));
     }
     else
     {

rigs/icom/icom.h

@@ -30,7 +30,7 @@
 #include <sys/time.h>
 #endif
 
-#define BACKEND_VER "20210316"
+#define BACKEND_VER "20210318"
 
 /*
  * defines used by comp_cal_str in rig.c