media: rcar-vin: Only dynamically allocate v4l2_async_subdev

In preparation of removing the usage of the old helper
v4l2_async_notifier_parse_fwnode_endpoints_by_port() do not dynamically
allocate the whole structure containing the parameters for the parallel
interface, instead only allocate the v4l2_async_subdev structure. There
is no functional change.

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