mt76: usb: introduce __mt76u_init utility routine

Introduce __mt76u_init unitility routine and move mt7615 usb bus ops
into mt7615 module in order to allow specifying driver specific
parameter.
This is a preliminary patch to add usb support to mt7921 driver.

Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>