staging: comedi: ni_labpc: remove unnecessary 'bustype' check

Only the ISA versions of the LabPC boards support DMA and the
devpriv->dma_chan is only set during the ISA board attach if
DMA is available. The extra check of the board->bustype is not
necessary in labpc_ai_cmd().

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>