staging: comedi: ni_labpc: remove unnecessary private data validation

The labpc_free_dma_chan() function validates the private data. Remove the
unnecessary validation in labpc_datach().

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>