Staging: iio: fix up world writable sysfs files.

You should not be able to write to sysfs files from any user.

Cc: Graf Yang <graf.yang@analog.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>