mt76: Add missing include of linux/module.h

MODULE_FIRMWARE() is used in usb_mcu.c and provided by linux/module.h,
but this header file is not directly included. This causes problems in
backports with some kernel versions.

Add the missing include.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>