rndis_wlan: clear cfg80211 scan on rndis_wlan_stop()

Scanning gets stuck if device is stopped when scan is active. Fix by
clearing/aborting cfg80211 scan on rndis_wlan_stop().

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