mt76: verify evt type in usb mcu response

Verify if evt field is set to EVT_CMD_DONE in usb mcu
response messages. This is a preliminary patch for usb_mcu layer
unification between mt76x0u and mt76x2u driver.

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