[media] v4l2-device: fix a macro definition

v4l2_device_unregister_subdev() wrongly uses "arg..." instead of "## arg"
in its body. Fix it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>