media: adv748x: Zero entire struct v4l2_fwnode_endpoint

The v4l2_fwnode_parse_endpoint() function can make use of defaults in
multiple bus types. To use this feature, all callers must zero the rest of
the fields of this struct, too. All other drivers appear to do that
already apart from this one.

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>