V4L/DVB (13457): s2250: subdev conversion

Convert the s2250 i2c driver to use v4l2 subdev interface.

Signed-off-by: Pete Eberlein <pete@sensoray.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>