mt76: add usb implementation of {wr,rd}_rp

Add USB implementation for read and write reg pair routines.
The actual implementation can use mcu related routines according to
MCU state

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