of_mtd: fix header file include guard

It seems the include guard was copied from of_net.h.

Signed-off-by: Philipp Rosenberger <philipp.rosenberger@xse.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/linux/of_mtd.h

@@ -7,7 +7,7 @@
  */
 
 #ifndef __LINUX_OF_MTD_H
-#define __LINUX_OF_NET_H
+#define __LINUX_OF_MTD_H
 
 #ifdef CONFIG_OF_MTD