spelling: s/usefull/useful/

Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/arch/xtensa/kernel/time.c b/arch/xtensa/kernel/time.c
index cb6e38e..937d81f 100644
--- a/arch/xtensa/kernel/time.c
+++ b/arch/xtensa/kernel/time.c
@@ -201,7 +201,7 @@
 	if ((signed long)(get_ccount() - next) > 0)
 		goto again;
 
-	/* Allow platform to do something usefull (Wdog). */
+	/* Allow platform to do something useful (Wdog). */
 
 	platform_heartbeat();