[media] cx23885: Silence unknown command warnings

I am seeing a constant stream of warnings on my cx23885 based card:
	cx23885_tuner_callback(): Unknown command 0x2.

Add a check in cx23885_tuner_callback to silence it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>