rndis_wlan: use cfg80211_wext_handler

Now that cfg80211 functions are added and wext converted to use wext-compat
functions, remove wext structures and disabled code.
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>