wifi: iwlwifi: mvm: rework debugfs handling

mac80211 added a new callback to add a vif debugfs.
Implement it instead of adding the debugfs directly,
which will make it properly preserved over switching
the vif from non-MLD/MLD and back.

This requires some rework so that we still have the
symlink but trust mac80211 to add/remove the debugfs.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20231004123422.818810e242e6.I805a28f9fbef5c52a3a575d04e7a6a909ecf9078@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>