bus: arm-ccn: Handle correctly no-more-cpus case

When migrating events the driver picks another cpu using
cpumask_any_but() function, which returns value >= nr_cpu_ids
when there is none available, not a negative value as the code
assumed. Fixed now.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>