KVM: arm64: Correct return value on host version downgrade attempt

Once the hypervisor negotiates the FF-A version with the host, it should
remain locked-in. However, it is possible to load FF-A as a module first
supporting version 1.1 and then 1.0.

Without this patch, the FF-A 1.0 driver will use 1.0 data structures to
make calls which the hypervisor will incorrectly interpret as 1.1 data
structures. With this patch, negotiation will fail.

This patch does not change existing functionality in the case where a
FF-A 1.2 driver is loaded after a 1.1 driver; the 1.2 driver will need
to use 1.1 in order to proceed.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Per Larsen <perlarsen@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>