mt76: mt7921: fix airtime reporting

Fix {tx,rx}_airtime reporting for mt7921 driver. Wrong register definitions
trigger a tx hangs before resetting airtime stats.

Fixes: 163f4d22 ("mt76: mt7921: add MAC support")
Tested-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/c05333be0e3e85a50a71bb2de81034fe425e3701.1615284335.git.lorenzo@kernel.org