mt76: move mt76x2u_mcu_deinit routine in mt76-usb module

Move mt76x2u_mcu_deinit routine in usb_mcu layer in order to
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>