V4L/DVB (11748): pvrusb2: Don't use the internal i2c client list

The i2c core used to maintain a list of client for each adapter. This
is a duplication of what the driver core already does, so this list
will be removed as part of a future cleanup. Anyone using this list
must stop doing so.

For pvrusb2, I propose the following change, which should lead to an
equally informative output. The only difference is that i2c clients
which are not a v4l2 subdev won't show up, but I guess this case is
not supposed to happen anyway.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>