In rig_set_split_mode force vfo to vfo_a when tx_vfo==vfo_b

https://github.com/Hamlib/Hamlib/issues/1046

src/rig.c

@@ -4165,6 +4165,7 @@ int HAMLIB_API rig_set_split_mode(RIG *rig,
     // some rigs exhibit undesirable flashing when swapping vfos in split
     // so we turn it off, do our thing, and turn split back on
     rx_vfo = vfo;
+    if (tx_vfo == RIG_VFO_B) rx_vfo = RIG_VFO_A;
 
     if (vfo == RIG_VFO_CURR && tx_vfo == RIG_VFO_B) { rx_vfo = RIG_VFO_A; }
     else if (vfo == RIG_VFO_CURR && tx_vfo == RIG_VFO_A) { rx_vfo = RIG_VFO_B; }