[media] vb2: reject output buffers with V4L2_FIELD_ALTERNATE

This is not allowed by the spec and does in fact not make any sense.
Return -EINVAL if this is the case.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>