cfg80211: fix disassociation warning due to misuse of wdev->current_bss

WARN_ON was triggered at mlme.c:213 when dissociating from an AP.

wdev->current_bss->pub.bssid should be used in place of
wdev->current_bss for BSSID comparison.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>