V4L/DVB: v4l2-subdev.h: Add support for enum_dv_preset

The op enum_dv_preset was missing in this header but is needed by subdev
drivers with DV functionality.

Signed-off-by: Mats Randgaard <mats.randgaard@tandberg.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>