mac80211: fix return from ieee80211_assoc_success

sparse pointed out that I made a mistake converting
the return value of ieee80211_assoc_success to bool,
this place should return false instead of one of the
enum values (which would be true).

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