mt76x2: introduce mt76x2_mac_load_tx_status routine

Add mt76x2_mac_load_tx_status routine since tx stats register map is
shared between usb and pci based devices but usb devices do not have a
tx stat irq line as pcie ones and it is necessary to load tx statistics
using a workqueue

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