drivers/firmware/psci: Assign @err directly in hotplug_tests()

The return value of down_and_up_cpus() can be assigned to @err directly.
With that, the useless assignment to @err with zero can be dropped.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20200630075943.203954-1-gshan@redhat.com


Signed-off-by: Will Deacon <will@kernel.org>