media: rcar-vin: Rework parallel firmware parsing

Rework the parallel firmware parsing code to not use the soon to be
removed v4l2_async_notifier_parse_fwnode_endpoints_by_port() helper. The
change only aims to prepare for the removing of the old helper and there
are no functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>