Update src/linklayer/ieee80211/mac/Ieee80211Mac.ned

The Ethernet (802.3) MTU (Maximum Transmission Unit) for data frames is 1500 bytes and the Wireless (802.11) MTU for data frames is 2304 bytes (frame payload size before encryption)
Merge pull request #6 from kmoza/patch-2

Update src/linklayer/ieee80211/mac/Ieee80211Mac.ned