mt76x2: fix tx power configuration for VHT mcs 9

Fix tx power configuration for VHT 1SS/STBC mcs 9 since
in MT_TX_PWR_CFG_{8,9} mcs 8,9 bits are GENMASK(21,16) and
GENMASK(29,24) while GENMASK(15,6) are marked as reserved

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>