mac80211: support for mesh interfaces in mac80211 data path

This changes the TX/RX paths in mac80211 to support mesh interfaces.
This code will be cleaned up later again before being enabled.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>