media: v4l2-fwnode: simplify v4l2_fwnode_parse_link

This helper was introduced before those helpers where awailable. Convert
it to cleanup the code and improbe readability.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>