[media] si470x: v4l2-compliance fixes

Just a few fixes for problems found after updating v4l2-compliance to check
the frequency band enumeration.

Note that the i2c driver doesn't fill in bus_info, but since I can't test that
driver I've decided not to fix that.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>