wifi: mac80211: properly set old_links when removing a link

In ieee80211_sta_remove_link, valid_links is set to
the new_links before calling drv_change_sta_links, but
is used for the old_links.

Fixes: cb71f1d1

 ("wifi: mac80211: add sta link addition/removal")
Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>