[PATCH] ieee80211: allow mtu bigger than 1500

Hi

this patch allow to set the mtu between 1500 and 2304 (max octets in an
MSDU) for devices using ieee80211 linux stack.

Signed-off-by: Matthieu CASTET <castet.matthieu@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>