wifi: mt76: mt7915: fix mcs value in ht mode

Fix the error that mcs will be reduced to a range of 0 to 7 in ht mode.

Fixes: 70fd1333 ("mt76: mt7915: rework .set_bitrate_mask() to support more options")
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>