wireless: max MSDU size for DMG networks

In the 802.11ad, aka DMG (Dynamic Multi-Gigabit), aka 60Ghz
spec, maximum MSDU size extended to 7920 bytes.
add #define for this.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>