staging: comedi: pcl818: fix pcl818_handle_dma() for short DMA transaction

Currently this function always transfers the full DMA buffer to the comedi
async buffer. When the cmd->stop_src == TRIG_COUNT the last DMA transfer
might be smaller than the buffer size. This results in invalid data being
added to the asunc buffer.

Add a 'size' member to the DMA descriptor and initialize it with the
actual size of the DMA transfer. Use that in pcl818_handle_dma() to
return the proper number of samples. Use the comedi_bytes_to_samples()
helper to convert the byte size to comedi samples. Remove the unnecessary
'bufptr' local variable.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>