coresight: etm4x: Fix unused function warning

Some of the newly added code in the etm4x driver is inside of an #ifdef,
and some other code is outside of it, leading to a harmless warning when
CONFIG_CPU_PM is disabled:

drivers/hwtracing/coresight/coresight-etm4x.c:68:13: error: 'etm4_os_lock' defined but not used [-Werror=unused-function]
 static void etm4_os_lock(struct etmv4_drvdata *drvdata)
             ^~~~~~~~~~~~

To avoid the warning and simplify the the #ifdef checks, use
IS_ENABLED() instead, so the compiler can drop the unused functions
without complaining.

Fixes: f188b5e7 ("coresight: etm4x: Save/restore state across CPU low power states")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[Fixed capital 'f' in title]
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20191213223107.1484-2-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm4x.c

@@ -1132,7 +1132,6 @@ static void etm4_init_trace_id(struct etmv4_drvdata *drvdata)
 	drvdata->trcid = coresight_get_trace_id(drvdata->cpu);
 }
 
-#ifdef CONFIG_CPU_PM
 static int etm4_cpu_save(struct etmv4_drvdata *drvdata)
 {
 	int i, ret = 0;
@@ -1402,17 +1401,17 @@ static struct notifier_block etm4_cpu_pm_nb = {
 
 static int etm4_cpu_pm_register(void)
 {
-	return cpu_pm_register_notifier(&etm4_cpu_pm_nb);
+	if (IS_ENABLED(CONFIG_CPU_PM))
+		return cpu_pm_register_notifier(&etm4_cpu_pm_nb);
+
+	return 0;
 }
 
 static void etm4_cpu_pm_unregister(void)
 {
-	cpu_pm_unregister_notifier(&etm4_cpu_pm_nb);
+	if (IS_ENABLED(CONFIG_CPU_PM))
+		cpu_pm_unregister_notifier(&etm4_cpu_pm_nb);
 }
-#else
-static int etm4_cpu_pm_register(void) { return 0; }
-static void etm4_cpu_pm_unregister(void) { }
-#endif
 
 static int etm4_probe(struct amba_device *adev, const struct amba_id *id)
 {