Allow TX to complete earlier

[tkerby]

Could we merge in these changes? matthijskooijman@6e09886

This allows the TX_Complete to happen earlier, thus letting a battery device sleep and save power yet heeps the holdoff on the next transmission intact.

[terrillmoore]

We could do something like this. But it's not needed at all unless the bandplan is doing uplink and downlink on the same channel. (In particular, it's wrong for the US and AU.) I'll put it on the list.