Add 100ms sleep to kenwood.c after set_ptt OFF

In kenwood.c when ? reply is receive write the transaction again instead of just reading
https://github.com/Hamlib/Hamlib/issues/644

rigs/kenwood/kenwood.c

@@ -450,7 +450,7 @@ transaction_read:
             {
                 rig_debug(RIG_DEBUG_ERR, "%s: Retrying shortly\n", __func__);
                 hl_usleep(rig->caps->timeout * 1000);
-                goto transaction_read;
+                goto transaction_write;
             }
 
             retval = -RIG_ERJCTED;
@@ -3757,6 +3757,8 @@ int kenwood_set_ptt(RIG *rig, vfo_t vfo, ptt_t ptt)
 
     int retval = kenwood_transaction(rig, ptt_cmd, NULL, 0);
 
+    if (ptt == RIG_PTT_OFF) hl_usleep(100*1000); // a little time for PTT to turn off
+
     RETURNFUNC(retval);
 }
 

rigs/kenwood/kenwood.h

@@ -28,7 +28,7 @@
 #include "token.h"
 #include "misc.h"
 
-#define BACKEND_VER "20210328"
+#define BACKEND_VER "20210402"
 
 #define EOM_KEN ';'
 #define EOM_TH '\r'