mt76x0: use mt76u_init for bus initialization

Use mt76u_init utility routine for usb initialization.
Moreover remove following unused routines:
- mt76x0_rr
- mt76x0_wr
- mt76x0_rmw
- mt76x0_wr_copy
- mt76x0_assign_pipes

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>