staging: comedi: das16: use comedi_async 'scans_done' to detect EOA

Refactor das16_interrupt() to use the comedi_async 'scans_done' member to
detect the End-of-Acquisition for the async command. Use the helper function
comedi_nsamples_left() to determine if DMA needs to be restarted.

This allows the removal of the private data 'adc_byte_count' and fixes a
possible integer overflow issue when that value is calculated.

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>