ppc64 iSeries: Make smp_release_cpus() callable on iSeries

We don't need to call smp_release_cpus() on iSeries but it's harmless
if we do and it removes another #ifdef ISERIES.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>