mt76x0: use mt76_register_device for device registration

Use mt76_register_device routine for device registration.
mt76_register_device allows to enable VHT support on 5GHz band.
Overwrite unsupported vht features with mt76x0_vht_cap_mask routine.
Remove macaddr field of mt76x0_dev data structure and
use the mt76_dev one. Moreover remove following unused routines:
- mt76_init_sband
- mt76_init_sband_2g
- mt76_init_sband_5g

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