cfg80211: don't request disconnect if not connected

Neil Brown reports that with libertas, my recent cfg80211
SME changes in commit ceca7b71
("cfg80211: separate internal SME implementation") broke
libertas suspend because it we now asked it to disconnect
while already disconnected.

The problematic change is in cfg80211_disconnect() as it
previously checked the SME state and now calls the driver
disconnect operation unconditionally.

Fix this by checking if there's a current_bss indicating
a connection, and do nothing if not.

Reported-and-tested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>