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

PPDU TxS can only report MPDU count whereas mac80211 requires MSDU scale
(NL80211_STA_INFO_TX_PACKETS), so switch to get MSDU counts from WA
statistic.

Note that mt7915 WA firmware only counts tx_packet for WED path, so driver
needs to take care of host path additionally.

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>