V4L/DVB (6273): V4L: vivi.c vidioc_try_fmt_cap() negotiate a valid field

If the client provides V4L2_FIELD_ANY vivi should return a valid field :)

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>