Staging: comedi: ni_65xx.c: fix output inversion problem.

When reading DO subdevice with inverted outputs invert the values read
back from the ports to match the inversion of values written.

Signed-off-by: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>