HID: wacom: make sysfs permissions more strict

It's not really dangerous in this driver, but it's against general
practice and worth fixing.

Wacom uses the attribute for changing the reporting speed of the
tablet (and this actually requires poking the device in the
background) (still I wouldn't consider it a security issue though).

udev is a proper place to handle this.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>