mt76x02: use mask for vifs

Use vif_mask to count interfaces to allow to set mac address in HW
if there is only one interface and report error if we create
interface with wrong BSSID resulting in already used index.

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