get_parent is supposed to return an unsigned 8 bit integer, so returning
-EINVAL is a bad idea.

Remove it.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NChen-Yu Tsai <wens@csie.org>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>