media: v4l2: clarify H.264 loop filter offset controls

The venus and s5p-mfc drivers add the loop filter alpha/beta offset
controls V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_ALPHA/BETA with a range of
-6 to +6, inclusive. This is exactly the range specified for the slice
header fields slice_alpha_c0_offset_div2 and slice_beta_offset_div2,
which store half the actual filter offsets FilterOffsetA/B.

Clarify that this control contains the halved offsets.

Signed-off-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>