staging: comedi: ni_660x: Add NI PCI-6608 to list of supported devices

Previously, only the PXI version of the NI-6608 board was supported.  This
change adds support for the PCI version as well.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>