mt76: mt76u: add {read/write}_extended utility routines

Introduce extended utility routines to read/write data o usb bus. New
devices (e.g. mt7663u) will rely on both upper and lower part of the
register address. Add ext parameter to mt76u_init signature in order to
reuse the code adding mt7663u support.

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