Change icom_get_usb_echo_off to return TIMEOUT when it occurs instead of RIG_OK

https://github.com/Hamlib/Hamlib/issues/509

rigs/icom/icom.c

@@ -715,6 +715,8 @@ int icom_get_usb_echo_off(RIG *rig)
 
     rig_debug(RIG_DEBUG_VERBOSE, "%s: ack_len=%d\n", __func__, ack_len);
 
+    if (retval == -RIG_ETIMEOUT) return retval;
+
     if (retval == RIG_OK)
     {
         rig_debug(RIG_DEBUG_VERBOSE, "%s: USB echo on detected\n",

rigs/icom/icom.h

@@ -30,7 +30,7 @@
 #include <sys/time.h>
 #endif
 
-#define BACKEND_VER "20210107"
+#define BACKEND_VER "20210113"
 
 /*
  * defines used by comp_cal_str in rig.c