The MTU on all Aironet products is 1522 (except old frimware versions where it was 1518)
This allows for dot1q tagging etc
It is a fixed and can not be changed.
This is larger than a standard ethernet MTU of 1500 and given that the wired side of a WLAN is ethernet this MTU should not need to be changed
What problems are you encountering and why do you need to change the MTU there maybe another solution