cfg80211: clear SSID on disconnect/no connection

The SME state machine in cfg80211 uses the SSID stored
in struct wireless_dev internally, but fails to clear
it in multiple places (when giving up on a connection
attempt and when disconnecting). This doesn't matter to
the SME state machine, but does matter for IBSS. Thus,
in those cases, clear the SSID to avoid messing up the
IBSS state machine.

Reported-by: Joerg Albert <jal2@gmx.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>