Add a little debug to rig.c

https://github.com/Hamlib/Hamlib/issues/539

src/rig.c

@@ -1373,6 +1373,8 @@ static int set_cache_freq(RIG *rig, vfo_t vfo, freq_t freq)
 
     if (vfo == RIG_VFO_CURR) { vfo = rig->state.current_vfo; }
 
+    rig_debug(RIG_DEBUG_TRACE, "%s: set vfo=%s to freq=%.0f\n", __func__, rig_strvfo(vfo), freq);
+
     switch (vfo)
     {
     case RIG_VFO_CURR:
@@ -1433,6 +1435,8 @@ static int get_cache_freq(RIG *rig, vfo_t vfo, freq_t *freq, int *cache_ms)
 
     if (vfo == RIG_VFO_CURR) { vfo = rig->state.current_vfo; }
 
+    rig_debug(RIG_DEBUG_TRACE, "%s: get vfo=%s\n", __func__, rig_strvfo(vfo));
+
     // VFO_C to be implemented
     switch (vfo)
     {