Actually that isn't true - as long as the upstream router is configured to send unreachables (ICMP Type 3, Code 4) i.e. Fragmentation Needed the AP SAP MTU gets set automatically.
I tested this by setting the IP MTU on an HP MSR router between an AP and VMC to 1400 bytes.
When 'ip unreachables enable' was configured, the router sends an ICMP message stating 'Fragmentation needed, MTU of next hop: 1400'.
show ap debug system-status ap-name TEST-CAP
MTU Discovery
-------------
IP Probes Responses Last Sent Last Rcvd jumbo:tx/rx 9KB: tx/rx
-- ------ --------- --------- --------- ----------- ----------
192.168.10.40 7 0 532 0 7/0 0/0
Switch MTU, Update sent time, Ack receive time, Current time (Time in seconds since AP booted)
1400 532 532 569
When unreachables was set disabled, there is no ICMP message from the router and the AP sets its MTU to 1500:
MTU Discovery
-------------
IP Probes Responses Last Sent Last Rcvd jumbo:tx/rx 9KB: tx/rx
-- ------ --------- --------- --------- ----------- ----------
192.168.10.40 2 1 225 225 2/0 0/0
Switch MTU, Update sent time, Ack receive time, Current time (Time in seconds since AP booted)
1500 225 225 228
There is still a failed probe for the initial GRE packet the AP sends with a length of 1578 (to find out if the network supports jumbo frames or not).