wifi: cfg80211: fix regulatory disconnect for non-MLO

The multi-link loop here broke disconnect when multi-link
operation (MLO) isn't active for a given interface, since
in that case valid_links is 0 (indicating no links, i.e.
no MLO.)

Fix this by taking that into account properly and skipping
the link only if there are valid_links in the first place.

Cc: stable@vger.kernel.org
Fixes: 7b0a0e3c

 ("wifi: cfg80211: do some rework towards MLO link APIs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20230616222844.eb073d650c75.I72739923ef80919889ea9b50de9e4ba4baa836ae@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>