wifi: mt76: connac: fix stats->tx_bytes calculation

The stats->tx_bytes shall subtract retry byte from tx byte.

Fixes: 43eaa368 ("wifi: mt76: add PPDU based TxS support for WED device")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/b3cd45596943cf5a06b2e08e2fe732ab0b51311b.1682285873.git.ryder.lee@mediatek.com