staging: comedi: ni_pcimio: Add support for NI PXIe-6251

Paul Fulmek reports that PXIe-6251 works the same as the existing
PCIe-6251 and just needs the new PCI device ID adding to ni_pci_table[]
and a new entry adding to ni_boards[] based on the existing entry for
PCIe-6251.

The new entry has PCI device ID 0x72e8 and board name "pxie-6251".

Thanks Paul!

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