membuff: Rename functions to have membuf_ prefix

The double 'f' is not necessary and is a bit annoying as elsewhere in
U-Boot we use 'buf'. Rename all the functions before it is used more
widely.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index 77a1558..cc0491b 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -63,7 +63,7 @@
 
 	if (state->term_raw != STATE_TERM_RAW)
 		disable_ctrlc(1);
-	membuff_init(&priv->buf, priv->serial_buf, sizeof(priv->serial_buf));
+	membuf_init(&priv->buf, priv->serial_buf, sizeof(priv->serial_buf));
 
 	return 0;
 }
@@ -138,15 +138,15 @@
 		return 0;
 
 	os_usleep(100);
-	avail = membuff_putraw(&priv->buf, 100, false, &data);
+	avail = membuf_putraw(&priv->buf, 100, false, &data);
 	if (!avail)
 		return 1;	/* buffer full */
 
 	count = os_read(0, data, avail);
 	if (count > 0)
-		membuff_putraw(&priv->buf, count, true, &data);
+		membuf_putraw(&priv->buf, count, true, &data);
 
-	return membuff_avail(&priv->buf);
+	return membuf_avail(&priv->buf);
 }
 
 static int sandbox_serial_getc(struct udevice *dev)
@@ -156,7 +156,7 @@
 	if (!sandbox_serial_pending(dev, true))
 		return -EAGAIN;	/* buffer empty */
 
-	return membuff_getbyte(&priv->buf);
+	return membuf_getbyte(&priv->buf);
 }
 
 #ifdef CONFIG_DEBUG_UART_SANDBOX