drivers: firmware: psci: Fix the expected return code from remove_cpu()

The PSCI checker disables CPUs in order to check the PSCI implementation.
One of its fun tricks, is to disable the only running CPU in the system,
this is expected to fail.

Since commit 93ef1429 ("cpu/hotplug: Add new {add,remove}_cpu()
functions"), the return code seen by this path can also be -EPERM from
offline_device(). psci_checker prints an error about an unexpected
return code in this case.

Fix the expected return code.

Fixes: 93ef1429 ("cpu/hotplug: Add new {add,remove}_cpu() functions")
Signed-off-by: James Morse <james.morse@arm.com>