mac80211: Use debugfs_create_xul() helper

Use the new debugfs_create_xul() helper instead of open-coding the same
operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Link: https://lore.kernel.org/r/20191025094130.26033-3-geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>