[media] af9015: use USB core soft_unbind

I think it is better to allow driver finish pending operations and
disconnect device. Killing all ongoing USB transfers could have very
bad effect, even break hardware in worst case.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>