[MAC80211]: split up __ieee80211_rx

The really indented part that does the huge switch on the interface
type is a nuisance. Put it into an own function 'prepare_for_handlers'.

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>