staging: comedi: das16: remove DMA irq support

As noted in the driver, a timer is always used by this driver to handle
the DMA because samples could be dropped while waiting for the DMA done
interrupt.

Remove the irq setup code as well as the interrupt handler.

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>