KVM: arm64: Match PMU error code descriptions with error conditions

Update the description of the PMU KVM_{GET, SET}_DEVICE_ATTR error codes
to be a better match for the code that returns them.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Link: https://lore.kernel.org/r/20200924123731.268177-3-alexandru.elisei@arm.com