powerpc/smp: Convert NR_CPUS to nr_cpu_ids

nr_cpu_ids can be limited by nr_cpus boot parameter, whereas NR_CPUS is a
compile time constant, which shouldn't be compared against during cpu kick.

Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>