mt76: mt7615: support 16 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.

Note that two PHYs share global 32 omac slots with each other.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>