staging: comedi: pcl816: fix DMA 'bytes' calculation

The calculation for the total number of bytes in the DMA transfer
is incorrect. Fix it.

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>