media: v4l2-ctrls.c: initialize an error return code with zero

Fix this smatch error:

drivers/media/v4l2-core/v4l2-ctrls.c:2971 v4l2_ctrl_request_clone() error: uninitialized symbol 'err'.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>