mt76: mt7915: support 32 station interfaces

When looking for a MAC address slot, start by using main BSSID slots 1-3,
afterwards use 16 repeater mode BSSID slots, then start using the slots
usually used for AP mode.
This search order should prevent unnecessary conflicts with AP mode interfaces
on the same PHY

Signed-off-by: Felix Fietkau <nbd@nbd.name>