arm64: hyp-stub: Zero x0 on successful stub handling

We now return HVC_STUB_ERR when a stub hypercall fails, but we
leave whatever was in x0 on success. Zeroing it on return seems
like a good idea.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>