binman: Call helper function binman_set_rom_offset() to fill offset

There is prepared function for filing rom_offset. That's why use it instead
of copying content of it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/binman.c

@@ -103,7 +103,7 @@ int binman_init(void)
 			return log_msg_ret("first image", -ENOENT);
 		binman->image = node;
 	}
-	binman->rom_offset = ROM_OFFSET_NONE;
+	binman_set_rom_offset(ROM_OFFSET_NONE);
-
+\
 	return 0;
 }