[MAC80211]: Add maintainers entry for mac80211.

Add MAINTAINERS entry for mac80211.

Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>