mac80211: Look out for some other AP when disassoc is received.

When a disassoc packet is received from the AP with a reason code of
'leaving the BSS', mac80211 should go into DISABLED state just as it
would do if the AP suddenly went away for some reason, as that is what
will happen shortly after the AP leaves anyway.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>