wifi: mac80211: mlme: refactor assoc link setup

Factor out the code to set up the assoc link into a
new function ieee80211_setup_assoc_link().

While at it, also modify the 'override' handling to
just take into account whether or not the conn_flags
were changed, which is what we need to setup again
the channel later.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>