staging: comedi: das1800: use comedi_async 'scans_done' to detect EOA

Remove the private data member 'count' and use the comedi_async 'scans_done'
member to detect the end-of-acquisition.

Use the comedi_nsamples_left() helper to get the number of samples to actually
add to the async buffer.

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>