staging: comedi: comedidev.h: add 'scans_done' member to comedi_async

Introduce a new member to comedi_async to count the number of scans completed.
This member is cleared by comedi_buf_reset() along with the other comedi_async
members. It is incremented in comedi_inc_scan_progress() when the end of scan
is detected.

This member will be used to clean up the scan counting in the comedi drivers.

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>