[media] sliced vbi: subdevs shouldn't clear the full v4l2_sliced_vbi_format struct

Various subdevs cleared the full v4l2_sliced_vbi_format struct, when
only the service_set/lines fields should have been cleared.
Due to this the io_size field was wrongly cleared to 0, causing a
v4l2-compliance error.

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