spelling: s/usefull/useful/

Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/drivers/w1/Kconfig b/drivers/w1/Kconfig
index 9a1e00d..4baf61a 100644
--- a/drivers/w1/Kconfig
+++ b/drivers/w1/Kconfig
@@ -3,7 +3,7 @@
 config W1
 	tristate "Dallas's 1-wire support"
 	---help---
-	  Dallas's 1-wire bus is usefull to connect slow 1-pin devices
+	  Dallas's 1-wire bus is useful to connect slow 1-pin devices
 	  such as iButtons and thermal sensors.
 
 	  If you want W1 support, you should say Y here.