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>

arch/microblaze/kernel/head.S

@@ -205,7 +205,7 @@ GT4: /* r11 contains the rest - will be either 1 or 4 */
 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 */