mt76: mt76u: rely on usb_interface instead of usb_dev

usb drivers are supposed to communicate using usb_interface instead
mt76x{0,2}u is now registering through usb_device. Fix it by passing
usb_intf device to mt76_alloc_device routine.

Fixes: 112f980a ("mt76usb: use usb_dev private data")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-By: Zero_Chaos <sidhayn@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>