ARM: perf: return NOTIFY_DONE from cpu notifier when no available PMU

When attempting to reset the PMU state for either a NULL PMU or a PMU
implementation without a reset function, return NOTIFY_DONE from the CPU
notifier as we don't care about the hotplug event.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm/kernel/perf_event_cpu.c

@@ -165,6 +165,8 @@ static int __cpuinit cpu_pmu_notify(struct notifier_block *b,
 
 	if (cpu_pmu && cpu_pmu->reset)
 		cpu_pmu->reset(cpu_pmu);
+	else
+		return NOTIFY_DONE;
 
 	return NOTIFY_OK;
 }