media: v4l2-dev: convert VFL_TYPE_* into an enum

Using enums makes easier to document, as it can use kernel-doc
markups. It also allows cross-referencing, with increases the
kAPI readability.

Please notice that now cx88_querycap() has to have a default for
the VFL type, as there are more types than supported by the driver.

Acked-By: Mike Isely <isely@pobox.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>