wifi: mac80211: mlme: fix override calculation

In my previous changes here, I neglected to take the old
conn_flags into account that might still be present from
the authentication, and thus ieee80211_setup_assoc_link()
can misbehave, as well as the override calculation being
wrong. Fix that by ORing in the old flags.

Fixes: 1845c1d4 ("wifi: mac80211: mlme: refactor assoc link setup")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>