mt76: connac: move mt76_connac_write_hw_txp in shared code

Now we can move mt76_connac_write_hw_txp routine in mt76-connac module
and reuse it in mt7921e and mt7615e driver.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>