media: v4l2-dev: fix WARN_ON(!vdev->device_caps)

Device nodes of type VFL_TYPE_SUBDEV have a 0 device_caps, so they
trigger this warning. Add a check against VFL_TYPE_SUBDEV.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 3c135050

 ("media: v4l2-dev/ioctl: require non-zero device_caps, verify sane querycap results")
Reported-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>