mt76: add multiple regs read support to usb_mcu layer

Introduce multiple regs read support to mcu layer in mt76-usb module.
Multiple regs read will be reused by mt76x0 driver in usb mcu layer
unification between mt76x0 and mt76x2u drivers

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