powerpc/powermac: Add missing of_node_put() in smp_core99_setup()

In smp_core99_setup(), add of_node_put() to drop the reference once it's
no longer needed.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220620150518.4074910-1-windhl@126.com