ssb: convert bus code to use dev_groups

The dev_attrs field of struct bus_type is going away soon, dev_groups
should be used instead.  This converts the ssb bus code to use the
correct field.

Cc: Michael Buesch <m@bues.ch>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>