[MAC80211]: reorder interface related functions

This patch groups a whole bunch of functions together to make
ieee80211.c more maintainable.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>