w1: use default attribute groups for w1 slave devices

As we have 2 sysfs files for the w1 slave devices, let the driver core
create / destroy them automatically by setting the default attribute
group for them, saving code and housekeeping logic.

Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>