Add support for the third loop filter mode
V4L2_MPEG_VIDEO_H264_LOOP_FILTER_MODE_DISABLED_AT_SLICE_BOUNDARY,
and fix V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_ALPHA and
V4L2_CID_MPEG_VIDEO_H264_LOOP_FILTER_BETA controls.

The filter offset controls are signed values in the -6 to 6 range and
are stored into the slice header fields slice_alpha_c0_offset_div2 and
slice_beta_offset_div2. The actual filter offsets FilterOffsetA/B are
double their value, in range of -12 to 12.

Rename variables to more closely match the nomenclature in the H.264
specification.
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>