mt76x0: usb: remove mt76_fw definition

Remove mt76_fw dependency from mt76x0u_upload_firmware routine since
it does not define firmware layout properly. Moreover use mt76_poll_msec
utility routine to check if the fw is properly running

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>