wifi: cfg80211: get correct AP link chandef

When checking for channel regulatory validity, use the
AP link chandef (and not mesh's chandef).

Fixes: 7b0a0e3c

 ("wifi: cfg80211: do some rework towards MLO link APIs")
Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>