staging: comedi: ni_labpc_isadma: tidy up labpc_init_dma_chan()

DMA support is optional for the labpc driver. The return value from
labpc_init_dma_chan() is not even checked by the caller.

Change the return type to void and tidy up the function a bit.

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>