media: vp8: Rename V4L2 VP8 specific macro to V4L2_VP8_

Before we move the VP8 controls to a stable public header,
rename this macro so it's clearly V4L2-specific.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>