cfg80211: check for current_bss from giwrate

When connecting to an ESSID manually, we may not set the BSSID, and thus
wdev->wext.connect.bssid will be NULL.
wdev->current_bss is always updated when a connection is established so we
should check it first.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>