[media] em28xx: fix kernel oops when watching digital TV

dvb->adapter.priv should also be set to the i2c bus since that's what
em28xx_tuner_callback expects.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>