mt76: mt76x02: add check for invalid vif idx

On adding vifs the idx can become 1 + (7 & 7) = 8 for APs.
Check against that, as only AP vif idx 0-7 is possible.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>