mac80211: bail out from prep_connection() if a reconfig is ongoing

If ieee80211_hw_restart() is called during authentication, the
authentication process will continue, causing the driver to be called
in a wrong state.  This ultimately causes an oops in the iwlwifi
driver (at least).

This fixes bugzilla 195299 partly.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=195299
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>