PM / hibernate: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This addresses Coverity-ID: 114713 ("Missing break in switch").
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/hibernate.c

@@ -638,6 +638,7 @@ static void power_down(void)
 		break;
 	case HIBERNATION_PLATFORM:
 		hibernation_platform_enter();
+		/* Fall through */
 	case HIBERNATION_SHUTDOWN:
 		if (pm_power_off)
 			kernel_power_off();