wifi: mt76: mt7921: Place upper limit on station AID

wifi: mt76: mt7921: Place upper limit on station AID

Any station configured with an AID over 20 causes a firmware crash.
This situation occurred in our testing using an AP interface on 7922
hardware, with a modified hostapd, sourced from Mediatek's OpenWRT
feeds.

In stock hostapd, station AIDs begin counting at 1, and this
configuration is prevented with an upper limit on associated stations.
However, the modified hostapd began allocation at 65, which caused the
firmware to crash. This fix does not allow these AIDs to work, but will
prevent the firmware crash.

This crash was only seen on IFTYPE_AP interfaces, and the fix does not
appear to have an effect on IFTYPE_STATION behavior.

Fixes: 5c14a5f944b9 ("mt76: mt7921: introduce mt7921e support")
Signed-off-by: Rory Little <rory@candelatech.com>
Link: https://patch.msgid.link/20250904000711.3033860-1-rory@candelatech.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>