staging: comedi: adl_pci9111: use comedi_async 'scans_done' to detect EOA

The comedi core now counts the number of samples added to the async buffer and
detects the end-of-scan and increments the comedi_async 'scans_done' counter.

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

This fixes a possible interger overflow when calculating the value of the
'stop_counter' and removes the need to accumulate the 'total' number of
samples added to the async buffer in pci9111_handle_fifo_half_full().

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>