mac80211: move sdata debugfs dir to vif

There is need create driver own per interface debugfs files. This is
currently done by drv_{add,remove}_interface_debugfs() callbacks. But it
is possible that after we remove interface from the driver (i.e.
on suspend) we call drv_remove_interface_debugfs() function. Fixing this
problem will require to add call drv_{add,remove}_interface_debugfs()
anytime we create and remove interface in mac80211. So it's better to
add debugfs dir dentry to vif structure to allow to create/remove
custom debugfs driver files on drv_{add,remove}_interface().

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>