cfg80211: remove WARN_ON in __cfg80211_sme_scan_done

cfg80211_sme_scan_done() can be called (by fullmac cards) with
wdev->conn == NULL when CFG80211_SME_CONNECTING. We quit silently
instead of WARN_ON in this case.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>