mt76: mt7921: fix ht mcs in mt7921_mac_add_txs_skb()

The mcs value of HT mode reported by mt7921_mac_add_txs_skb()
has already been converted to the expected format.

Fixes: 970ab80e

 ("mt76: mt7921: report tx rate directly from tx status")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>