clk: sunxi: tcon-ch1: Do not return a negative error in get_parent

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

Remove it.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>