media: v4l2-fwnode: Allow passing NULL fwnode to endpoint parsers

media: v4l2-fwnode: Allow passing NULL fwnode to endpoint parsers

v4l2_fwnode_endpoint_parse() and v4l2_fwnode_endpoint_alloc_parse() take
a fwnode as the first argument and leave it up to the caller to check a
valid fwnode has been obtained through various means. Instead, add a check
here so the callers won't need to do that anymore.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Michael Riesch <michael.riesch@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>