wifi: mac80211: add API to show the link STAs in debugfs

Create debugfs data per-link. For drivers, there is a new operation
link_sta_add_debugfs which will always be called.

For non-MLO, the station directory will be used directly rather than
creating a corresponding subdirectory. As such, non-MLO drivers can
simply continue to create the data from sta_debugfs_add.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
[add missing inlines if !CONFIG_MAC80211_DEBUGFS]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>