staging: comedi: ni_660x: Support PCI-6224

Add support for the NI PCI-6224 board, assuming it behaves like the NI
PXI-6224 board at the register level.

The PCI device ID comes from the "nitiowv.inf" file in National
Instrument's Windows drivers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>