Staging: comedi: range.c: properly mark up __user pointers

This is the start of cleaning up the user pointer markings
in the comedi core.

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