io_fip: return -ENFILE when a file is already open

The cause of failure is not memory shortage.

The comment for ENFILE in include/lib/libc/errno.h

  /* Too many open files in system */

... is a better match to the warning message here.

Change-Id: I45a1740995d464edd8b3e32b93f1f92ba17e5874
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drivers/io/io_fip.c

@@ -302,7 +302,7 @@ static int fip_file_open(io_dev_info_t *dev_info, const uintptr_t spec,
 	 */
 	if (current_file.entry.offset_address != 0) {
 		WARN("fip_file_open : Only one open file at a time.\n");
-		return -ENOMEM;
+		return -ENFILE;
 	}
 
 	/* Attempt to access the FIP image */