• Add in the AR7240 global control field for setting the maximum frame size for the AR7240.

• Include SM/MS macros, thanks to ath_hal(4).

• This field is for normal packets, VLAN and other headers are added to this by the switch device.

• Set the MTU to 1536, to match what is done in Linux. Use the SM macro to write this field.

Obtained from: Atheros (AR7240 datasheet), Linux OpenWRT (MTU default)