staging: comedi: amplc_pci230: use comedi_async 'scans_done' to detect AO EOA

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

Use the helper function comedi_nscans_left() to get the number of scans in the
async buffer of left in the command.

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>