ARM: smp: fix clipping of number of CPUs

Rather than clipping the number of CPUs using the compile-time NR_CPUS
constant, use the runtime nr_cpu_ids value instead.  This allows the
nr_cpus command line option to work as expected.

Cc: <stable@kernel.org>
Reported-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>