Before, only frames with a maximum size of 1528 bytes could be
transmitted between two 802.11s nodes.
For batman-adv for instance, which adds its own header to each frame,
we typically need an MTU of at least 1532 bytes to be able to transmit
without fragmentation.
This patch now increases the maxmimum frame size from 1528 to 1656
bytes.
Tested with two ath10k devices in 802.11s mode, as well as with
batman-adv on top of 802.11s with forwarding disabled.
Fix originally found and developed by Ben Greear.
