PM / OPP: Fix build when CONFIG_PM_OPP is not set

Commit 03ca370fbf7b76d6d002380dbdc2cdc2319f9c80 (PM / OPP: Add
OPP availability change notifier) does not compile if CONFIG_PM_OPP
is not set:

arch/arm/plat-omap/omap-pm-noop.o: In function `opp_get_notifier':
include/linux/opp.h:103: multiple definition of `opp_get_notifier'
include/linux/opp.h:103: first defined here

Also fix incorrect comment.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

include/linux/opp.h

@@ -97,11 +97,11 @@ static inline int opp_disable(struct device *dev, unsigned long freq)
 	return 0;
 }
 
-struct srcu_notifier_head *opp_get_notifier(struct device *dev)
+static inline struct srcu_notifier_head *opp_get_notifier(struct device *dev)
 {
 	return ERR_PTR(-EINVAL);
 }
-#endif		/* CONFIG_PM */
+#endif		/* CONFIG_PM_OPP */
 
 #if defined(CONFIG_CPU_FREQ) && defined(CONFIG_PM_OPP)
 int opp_init_cpufreq_table(struct device *dev,