staging: comedi: usbduxsigma: use comedi_async 'scans_done' to detect AI EOA

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

Move the EOA check so it happens after adding the samples from the current
urb to the async buffer. This prevents the unnecessary resubmit of the urb
when the EOA occurs.

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>