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.