[media] bttv: add VIDIOC_DBG_G_CHIP_IDENT

VIDIOC_DBG_G_CHIP_IDENT is a prerequisite for the G/S_REGISTER ioctls.
In addition, add support to call G/S_REGISTER for supporting i2c devices.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>