mt76: usb: remove unneeded {put,get}_unaligned

Compiler give us guarantees on variables alignment, so use
an variable as buffer when read/write registers and remove
unneeded {put,get}_unaligned.

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