microblaze: Fix typo in head.S s/substract/subtract/

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Cc: Michal Simek <monstr@monstr.eu>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/microblaze/kernel/head.S b/arch/microblaze/kernel/head.S
index 17645b2..4655ff3 100644
--- a/arch/microblaze/kernel/head.S
+++ b/arch/microblaze/kernel/head.S
@@ -205,7 +205,7 @@
 GT16: /* TLB0 is 16MB */
 	addik	r9, r0, 0x1000000 /* means TLB0 is 16MB */
 TLB1:
-	/* must be used r2 because of substract if failed */
+	/* must be used r2 because of subtract if failed */
 	addik	r2, r11, -0x0400000
 	bgei	r2, GT20 /* size is greater than 16MB */
 	/* size is >16MB and <20MB */