staging: comedi: das16: use COMEDI_CB_CANCEL_MASK to see if command is running

In das16_interrupt(), use COMEDI_CB_CANCEL_MASK to determine if the async
command is still running and the dma needs to be re-enabled. This will
cause the driver not re-enable the dma if the async buffer overflows
(COMEDI_CB_OVERFLOW), a hardware error occurs (COMEDI_CB_ERROR), or the
command completes (COMEDI_CB_EOA).

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>