staging: comedi: adv_pci1710: use comedi_async 'scans_done' to detect EOA

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

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>