cfg80211: fix disassoc while not associated

When trying to disassociate while not associated,
the kernel would crash rather than refusing the
operation, fix this;

Reported-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>