Staging: comedi: add ni_mio_common code

Common code for DAQ-STC based boards.  Other drivers #include
this file to take advantage of the functions here.

From: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>