net: lantiq_etop: add missing comment for wmb()

This patch adds the missing code comment for memory barrier
call and fixes checkpatch warning:

WARNING: memory barrier without comment
+	wmb();

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://lore.kernel.org/r/20211228214910.70810-1-olek2@wp.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>