wifi: mt76: mt7996: check txs format before getting skb by pid

The PPDU TXS does not include the error bit so it cannot use to report
status to mac80211. This patch fixes issue that STA wrongly detects if AP
is still alive.

Fixes: 2569ea53

 ("wifi: mt76: mt7996: enable PPDU-TxS to host")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>