media: mediatek: vcodec: Skip non CBR bitrate mode

V4L2_MPEG_VIDEO_BITRATE_MODE_CBR is the only bitrate mode supported
by the mediatek driver. The other bitrates must be skipped in
QUERY_MENU.

Fixes: d8e8aa86

 ("media: mediatek: vcodec: Report supported bitrate modes")
Signed-off-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>