mt76: mt7915: fix ht mcs in mt7915_mac_add_txs_skb()

The mcs value of HT mode reported by mt7915_mac_add_txs_skb()
has already been converted to the expected format.

Fixes: 9908d98a

 ("mt76: mt7915: 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>