media: uapi: vp8: Rename v4l2_vp8_loopfilter to v4l2_vp8_loop_filter

Rename to struct v4l2_vp8_loop_filter, which is closer to the
VP8 syntax. This change is only cosmetics, but since this API
is exposed to userspace, it's worth the trouble to get it consistent.

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>