Fix more cppcheck warnings in tmd710.c

kenwood/tmd710.c

@@ -2114,7 +2114,8 @@ int tmd710_vfo_op(RIG *rig, vfo_t vfo, vfo_op_t op)
 int tmd710_get_level(RIG *rig, vfo_t vfo, setting_t level, value_t *val)
 {
     char buf[10], ackbuf[20];
-    int retval, v, l;
+    int retval, v;
+    unsigned int l;
     int vfonum;
 
     rig_debug(RIG_DEBUG_TRACE, "%s: called\n", __func__);