KVM: arm64: Return a bool from emulate_cp()

KVM indicates success/failure in several ways, but generally an integer
is used when conditionally bouncing to userspace is involved. That is
not the case from emulate_cp(); just use a bool instead.

No functional change intended.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220503060205.2823727-2-oupton@google.com