Fix TXCONT continuous mode for Sx126x

[matthijskooijman]

In 2.2, some changes were made by Semtech to continous mode that seem to be applied only for the 127x. I quickly applied a small part of these changes to at least make compilation work again, but TXCONT mode is now probably implemented incorrectly for 126x. It is also not actually used by any code right now, I think, so this is not a big deal, but might need to be fixed at some point.

See b83bb94 for details.