wifi: iwlwifi: mvm: add a per-link debugfs

Add a per-link debugfs entry in iwlmvm level so we can read/write link
related parameters.
Do it by implementing the link_add_debugfs API introduced by mac80211.

Each entry will have a path like this:
.../netdev:wlan0/link-X/iwlmvm/<file_name>
for each link X.

For non-MLD vifs this callback is called when the original vif
debugfs dir is also created, so handle that case by not creating
the 'iwlmvm' directory again.

Note that we don't have to worry about the cleaning the iwlmvm/*
directory as it is already done by mac80211 when removing the link
(or netdev).

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/20231004123422.6a161f021ae8.Ic8f40f2b4682270c94036e3c11c3996ae34266fa@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>