staging: comedi: pcl812: simplify the dma->size calculations

The dma->size calculations are overly complicated in this driver and the
'prealloc_bufsz' sanity checks are unnecessary.

Currently this driver determines the number of DMA "runs" needed and the size
of the "last" DMA transfer in order to perform a command. As long as there are
more "runs" required, the dma->size is set to the buffer maxsize. On the last
"run" the buffer is set to the "last" size.

Refactor the driver to use the comedi core helpers to determine the DMA size
based on the buffer maxsize and the number of samples remaining in the command.

This allows removing the 'dma_runs_to_end' and 'last_dma_run' mamebers from
the private data.

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>