staging: comedi: dmm32at: use comedi_async 'scans_done' to detect EOA

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

This also removes the artifical max limit on the cmd->stop_arg.

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>