wifi: mac80211: mlme: refactor assoc success handling

Refactor the per-link setup out of ieee80211_assoc_success()
into a new function ieee80211_assoc_config_link().

It looks useless for now to parse the elements again inside
ieee80211_assoc_config_link(), but that will be done with
the link ID in the future.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>