staging: comedi: s626: use comedi_async 'scans_done' to detect EOA

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

Also, remove the unnecessary COMEDI_CB_EOS event. The core automatically
detects and adds that event.

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>