wifi: mt76: mt7915: rework testmode tx antenna setting

Let the configuration of testmode antenna mask on both band0 and band1
become antenna_mask rather than chainmask. This could simplify the
settings for user and get rid of the conversion when sending fw command.

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