wifi: mt76: mt7915: rework tx bytes counting when WED is active

Concurrent binding/non-binding skbs could be handled anywhere which leads
to mixed byte counting, so switch to use PPDU TxS reporting regardless Tx
paths when WED is active.

Fixes: 43eaa368

 ("wifi: mt76: add PPDU based TxS support for WED device")
Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>