media: venus: fix reported size of 0-length buffers

The last buffer is often signaled by an empty buffer with the
V4L2_BUF_FLAG_LAST flag set. Such buffers were returned with the
bytesused field set to the full size of the OPB, which leads
user-space to believe that the buffer actually contains useful data. Fix
this by passing the number of bytes reported used by the firmware.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>