mt76x2: add buffer len to mt76x2_mac_write_txwi signature

Add frame length to mt76x2_mac_write_txwi routine signature and do not
fetch it from skb since txwi data structure is added at the beginning of
the skb for usb based devices and mt76x2_mac_write_txwi will be shared
between pci and usb related code

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>