mt76usb: use synchronous msg for mcu command responses

Use usb_bulk_msg for reading MCU command responses. This simplify code
a lot.

Together with 97a3005759c ("mt76usb: allow mt76u_bulk_msg be used
for reads") it also fix possible problems with rx data buffers
not being aligned and contained within single page. After doing
page_frag_alloc(1024) consecutive page_frag_alloc(PAGE_SIZE) will
alloc PAGE_SIZE buffer at PAGE_SIZE - 1024 offset.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>