ARM: cpuidle: Pass on arm_cpuidle_suspend()'s return value

arm_cpuidle_suspend() may return -EOPNOTSUPP, or any value returned
by the cpu_ops/cpuidle_ops suspend call. arm_enter_idle_state() doesn't
update 'ret' with this value, meaning we always signal success to
cpuidle_enter_state(), causing it to update the usage counters as if we
succeeded.

Fixes: 191de17a ("ARM64: cpuidle: Replace cpu_suspend by the common ARM/ARM64 function")
Signed-off-by: James Morse <james.morse@arm.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: 4.1+ <stable@vger.kernel.org> # 4.1+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>