wifi: mt76: mt7996: get tx_retries and tx_failed from txfree

Retrieve tx retries/failed counts from 'txfree done' events and report
them via mt7996_sta_statistics().

Signed-off-by: Yi-Chia Hsieh <yi-chia.hsieh@mediatek.com>
Signed-off-by: Money Wang <Money.Wang@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>