membuf: Rename struct

Rename the struct to match the function prefix and filenames.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/serial.h b/arch/sandbox/include/asm/serial.h
index 16589a1..4150634 100644
--- a/arch/sandbox/include/asm/serial.h
+++ b/arch/sandbox/include/asm/serial.h
@@ -44,7 +44,7 @@
  * @buf: holds input characters available to be read by this driver
  */
 struct sandbox_serial_priv {
-	struct membuff buf;
+	struct membuf buf;
 	char serial_buf[16];
 	bool start_of_line;
 };