media: staging: rkisp1: fix test of return value of media_entity_get_fwnode_pad

media_entity_get_fwnode_pad returns negative value on error
and the pad numeber on success. Therefore change the error test
from 'if (ret)' to 'if (ret < 0)' .

Fixes: d65dd852 ("media: staging: rkisp1: add Rockchip ISP1 base driver")
Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>