staging: comedi: das1800: reduce indent level of das1800_init_dma()

An IRQ and at least one DMA channel are required to use DMA with this driver.
Move the check to the caller to reduce the indent level of this function.

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>