mt76: mt7615: rework chainmask handling

Move chainmask to struct mt7615_phy and instead of needlessly making the
format similar to values for older chips, make it refer to the actual chain
bits used for the rx/tx path.
This is important for multiple wiphy support, where for a secondary phy,
antenna_mask will start at 0, and chainmask will start at the chain offset
(bit 2)

Signed-off-by: Felix Fietkau <nbd@nbd.name>