mt76: mt7615: improve wmm index allocation

Typically all AP interfaces on a PHY will share the same WMM settings, while
sta/mesh interfaces will usually inherit the settings from a remote device.
In order minimize the likelihood of conflicting WMM settings, make all AP
interfaces share one slot, and all non-AP interfaces another one.

This also fixes running multiple AP interfaces on MT7613, which only has 3
WMM slots.

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