PPC: Fix powerpc vio_find_name to not use devices_subsys

This fixes vio_find_name() in arch/powerpc/kernel/vio.c, which is
currently broken because it tries to use devices_subsys.  That is bad
for two reasons: (1) it's doing (or trying to do) a scan of all
devices when it should only be scanning those on the vio bus, and
(2) devices_subsys was an internal symbol of the device system code
which was never meant for external use and has now gone away, and
thus the kernel fails to compile on pSeries.

The new version uses bus_find_device_by_name() on the vio bus
(vio_bus_type).

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>