wifi: iwlwifi: mld: only create debugfs symlink if it does not exist

wifi: iwlwifi: mld: only create debugfs symlink if it does not exist

When mac80211 switches between non-MLO and MLO it will recreate the
debugfs directories. This results in the add_if_debugfs handler being
called multiple times. As the convenience symlink is created in the mld
debugfs directory and not the vif, it will not be removed by mac80211
when this happens and still exists.

Add a check and only create the convenience symlink if we have not yet
done so.

Fixes: d1e879ec600f ("wifi: iwlwifi: add iwlmld sub-driver")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250420095642.2490696f032a.I74319c7cf18f7e16a3d331cb96e38504b9fbab66@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>