wifi: mt76: mt7996: fix header translation logic

When header translation failure is indicated, the hardware will insert
an extra 2-byte field containing the data length after the protocol
type field. This happens either when the LLC-SNAP pattern did not match,
or if a VLAN header was detected.

The previous commit accidentally breaks the logic, so reverts back.

Fixes: 27db47ab

 (wifi: mt76: mt7996: enable mesh HW amsdu/de-amsdu support)
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>