Add a callback which the driver can use to add the vif debugfs.
We used to have this back until commit d260ff12 ("mac80211:
remove vif debugfs driver callbacks") where we thought that it
will be easier to just add them during interface add/remove.

However, now with multi-link, we want to have proper debugfs
for drivers for multi-link where some files might be in the
netdev for non-MLO connections, and in the links for MLO ones,
so we need to do some reconstruction when switching the mode.

Moving to this new call enables that and MLO drivers will have
to use it for proper debugfs operation.
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/20230928172905.ac38913f6ab7.Iee731d746bb08fcc628fa776f337016a12dc62ac@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>