exynos4: Use EXYNOS4210_NCPUS instead of max_cpus on error message

The code is checking smp_cpus against EXYNOS4210_NCPUS, not against
max_cpus, so use EXYNOS4210_NCPUS in the error message for consistency.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
