flash: Rename header guard

Make it align with the file name

Change-Id: I304baec9edbc62470475e383d6f93640b25a0763
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/flash.h b/flash.h
index 3cfaef9..acf8218 100644
--- a/flash.h
+++ b/flash.h
@@ -1,8 +1,8 @@
 /* SPDX-License-Identifier: Apache-2.0 */
 /* Copyright (C) 2018 IBM Corp. */
 
-#ifndef MBOXD_FLASH_H
-#define MBOXD_FLASH_H
+#ifndef FLASH_H
+#define FLASH_H
 
 #define FLASH_DIRTY	0x00
 #define FLASH_ERASED	0x01
@@ -26,4 +26,4 @@
 #ifdef __cplusplus
 }
 #endif
-#endif /* MBOXD_FLASH_H */
+#endif /* FLASH_H */