spelling fixes: arch/um/

Spelling fixes in arch/um/.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
diff --git a/arch/um/drivers/null.c b/arch/um/drivers/null.c
index 21ad3d7..2b45a14 100644
--- a/arch/um/drivers/null.c
+++ b/arch/um/drivers/null.c
@@ -9,7 +9,7 @@
 #include "chan_user.h"
 #include "os.h"
 
-/* This address is used only as a unique identifer */
+/* This address is used only as a unique identifier */
 static int null_chan;
 
 static void *null_init(char *str, int device, const struct chan_opts *opts)