KVM: arm64: Fix comment on barrier in kvm_psci_vcpu_on()

The barrier is there for power_off rather than power_state.
Probably typo in commit 358b28f0

 ("arm/arm64: KVM: Allow
a VCPU to fully reset itself").

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211208193257.667613-3-tabba@google.com