cfg80211: put wext data into substructure

To make it more apparent in the code what is for wext
only (and needs to be #ifdef'ed) put all the info for
wext into a substruct in each wireless_dev.

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