rndis_wlan: add cfg80211 connect, disconnect, join_ibss and leave_ibss

Add cfg80211 connect functions for station and ad-hoc modes and
convert wext to use theim.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>