mac80211: act upon and report deauth while associating

When trying to associate, the AP could send a deauth frame instead.
Currently mac80211 drops that frame and doesn't report it to the
supplicant, which, in some versions and/or in certain circumstances
will simply keep trying to associate over and over again instead of
trying authentication again.

Fix this by reacting to deauth frames while associating, reporting
them to the supplicant and dropping the association attempt (which
is bound to fail.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>