rndis_wlan: convert get/set mode to cfg80211

Signed-off-by: John W. Linville <linville@tuxdriver.com>
[edit: made rndis_change_virtual_intf static]
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>