staging: comedi: ni_660x: support NI PXI-6624

Florent Boudet reports success using a NI PXI-6624 board with a
trivially modified version of the "ni_660x" driver (addition to the PCI
device ID table and comedi board table).  He did this with the
out-of-tree Comedi drivers at www.comedi.org, but it applies equally to
the in-tree "staging" drivers.

He reports the PXI-6624 is basically the same as the PXI-6602, but with
isolated channels and external voltage source.

Add support for NI PXI-6224 to the "ni_660x" driver.

(Maybe the driver should be renamed to "ni_66xx"?)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Florent Boudet <flboudet@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>