media: v4l2-fwnode: Avoid using PTR_ERR(NULL)

PTR_ERR(NULL) yields 0 which is commonly used to denote success. This is
the case here, and PTR_ERR(NULL) is apparently shunned upon. Fix this by
explicitly returning 0 if fwnode == NULL.

Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>