rndis_wlan: replace wireless_send_event with cfg80211_disconnected

Remove (hopefully) last use of WEXT in rndis_wlan. Replace wireless_send_event
with missing cfg80211_disconnected in rndis_wlan_do_link_down_work.

Reported-by: "Rogério Brito" <rbrito@ime.usp.br>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>