[POWERPC] Simplify vio_bus_init a little for legacy iSeries

iSeries_vio_dev was already statically initialised and we can remove
one set of #ifdef CONFIG_PPC_ISERIES guards.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>