drivers/hid/hid-picolcd_fb: avoid world-writable sysfs files.

In line with practice for module parameters, we're adding a build-time
check that sysfs files aren't world-writable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Bruno Prémont <bonbons@linux-vserver.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>