arm64: efi: fix initcall return values

Even though initcall return values are typically ignored, the
prototype is to return 0 on success or a negative errno value on
error. So fix the arm_enable_runtime_services() implementation to
return 0 on conditions that are not in fact errors, and return a
meaningful error code otherwise.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>