cfg80211: split wext compatibility to separate header

A lot of drivers erroneously use wext constants
and don't notice since cfg80211.h includes them.
Make this more split up so drivers needing wext
compatibility from cfg80211 need to explicitly
include that from cfg80211-wext.h.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>