cfg80211: remove warning in deauth case

It might be the case that __cfg80211_disconnected() has already
cleaned up wdev->current_bss() for us. The old code didn't catch
that situation and didn't warn needlessly.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>