[media] vivi: fix a few format-related compliance issues

This patch will always set the field to INTERLACED (this fixes a bug were a driver should
never return FIELD_ANY), and will default to YUYV pixelformat if an unknown pixelformat
was specified.

This way S/TRY_FMT will always return a valid format struct.

Regards,

	Hans

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>