Fix vtx_freq default value when USE_VTX_TABLES is defined

Fix vtx_freq default value when USE_VTX_TABLES is defined
When a `vtxDevice` was active (like if `USE_VTX_RTC6705` is defined for the target) then the initialization logic would try to look up the frequency based on the band/channel and update the `vtx_freq` setting. The problem is that initially the `vtxtable` is not defined so the frequency lookup fails and returns 0. This resulted in a defaults difference since the default value was 5740 which would have matched the default band and channel if the "standard" bands/channels were set up with `vtxtable`. So instead set the default value to 0 unless legacy built-in frequencies are used (`USE_VTX_TABLES` not defined).