wifi: mt76: mt7996: add support for variants with auxiliary RX path

Add support to correctly configure the rx chainmask of variants that
have additional auxiliary RX path. e.g., 4T5R.
The auxiliary RX path is transparent to driver, but driver needs to
correctly configure it in the set channel fw command.

Co-developed-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>