media: uapi: h264: Rename pixel format

The V4L2_PIX_FMT_H264_SLICE_RAW name was originally suggested
because the pixel format would represent H264 slices without any
start code.

However, as we will now introduce a start code menu control,
give the pixel format a more meaningful name, while it's
still early enough to do so.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>