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

In the Linux kernel, the following vulnerability has been resolved: 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.