staging: comedi: das16: "timer_mode" is always used for DMA

As noted in the driver, a timer is always used by this driver to handle the
DMA due to buggy DMA controllers and the lack of a hardware fifo in some of
the supported boards.

Remove all the non-timer mode specific code.

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>