[MAC80211]: split out some key functions from ieee80211.c

into a new file key.c which doesn't have much code right now but
it makes ieee80211.c easier to read.

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>