mt76usb: use usb_dev private data

Setup usb device private data. This allows to remove mt76u_buf->dev
and simplify some routines as no longer we need to get usb device
through usb interface.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>