Revert "Remove rig_flush from icom/frame.c"

This reverts commit ca3f19fbd6.
Evidence showing this is needed when sequencing is lost
Instead we need to improved Windows serial_flush

rigs/icom/frame.c

@@ -158,8 +158,7 @@ int icom_one_transaction(RIG *rig, unsigned char cmd, int subcmd,
     set_transaction_active(rig);
 
 collision_retry:
-    // does not seem necessary and add 6ms overhead per call in Windows
-    //rig_flush(&rs->rigport);
+    rig_flush(&rs->rigport);
 
     if (data_len) { *data_len = 0; }