media: v4l2-fwnode: v4l2_fwnode_endpoint_parse caller must init vep argument

Document that the caller of v4l2_fwnode_endpoint_parse() must init the
fields of struct v4l2_fwnode_endpoint (vep argument) fields.

It used to be that the fields were zeroed by v4l2_fwnode_endpoint_parse
when bus type was set to V4L2_MBUS_UNKNOWN but with recent changes (Fixes:
line below) that no longer makes sense.

Fixes: bb4bba92 ("media: v4l2-fwnode: Make bus configuration a struct")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>