PPC: VIO: convert bus code to use bus_groups

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

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>