staging: comedi: ni_labpc: allow board to attach without dma

If the dma channel is not available this driver will still work
with interrupts for the analog input subdevice command support.

In addition, only dma channels 1 and 3 are valid for the ISA
devices.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>