ANDROID: KVM: arm64: Fix MMIO guard unmap

Pierre-Clément reports that the MMIO guard unmap hypercall exposed to
protected guests returns an error upon success. Indeed,
SMCCC_RET_SUCCESS is returned only if __pkvm_remove_ioguard_page()
failed, which doesn't match the expected behaviour.

Fix this by returning SMCCC_RET_INVALID_PARAMETER instead.

Bug: 251426790
Reported-by: Pierre-Clément Tosi <ptosi@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: Id746fa7d5d3a03ee5df6d114a07240822a0be93b