media: videodev2.h: Move HI240 format to vendor-specific section

V4L2_PIX_FMT_HI240 is a 8-bit dithered RGB format specific to BTTV. Move
it from the packed YUV formats section where it was misplaced to the
vendor-specific formats section.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>