V4L/DVB (11159): pvrusb2: Providing means to stop tracking an old i2c module

This implements a temporary mechanism to "untrack" an i2c module from
the old i2c layer.  The v4l2-subdev related code in the driver will
use this to remove a sub-device from the old i2c layer.  In the end,
once the old i2c layer is removed, this will also eventually go away.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>