cfg80211: deprecate CFG80211_WEXT

Almost all wireless tools have transitioned to
or at least added compatibility with nl80211 so
there's no real need for CONFIG_CFG80211_WEXT
any more. Mark it for removal, and also change
the default to not be enabled.

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