PM: Constify returned PM event name

The pm_verb() returns a pointer to string from .rodata so it should be
marked as const.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/main.c

@@ -62,7 +62,7 @@ static pm_message_t pm_transition;
 
 static int async_error;
 
-static char *pm_verb(int event)
+static const char *pm_verb(int event)
 {
 	switch (event) {
 	case PM_EVENT_SUSPEND: