Change rig_set_split_mode to return RIG_OK when set_vfo and vfo_op are not available

For the FT-991 example there's no need to set mode on VFOB as there is only one mode for both VFOs
So if rig has neither of these we will assume VFOB mode does not need to be set
https://github.com/Hamlib/Hamlib/issues/684

src/rig.c

@@ -3879,7 +3879,8 @@ int HAMLIB_API rig_set_split_mode(RIG *rig,
     }
     else
     {
-        RETURNFUNC(-RIG_ENAVAIL);
+        rig_debug(RIG_DEBUG_WARN, "%s: rig does not have set_vfo or vfo_op. Assuming mode already set\n", __func__); 
+        RETURNFUNC(RIG_OK);
     }
 
     if (retcode != RIG_OK)