mt76: connac: move mt76_connac_chan_bw in common code

Move mt76_connac_chan_bw in mt76-connac code and remove
duplicated code from mt7921 and mt7915 driver.

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