serial: embed the rx buffer in struct serial_dev_priv

The initialization of upriv->buf doesn't check for a NULL return. But
there's actually no point in doing a separate, unconditional malloc()
in post_probe; we can just make serial_dev_priv contain the rx buffer
itself, and let the (larger) allocation be handled by the driver core
when it allocates the ->per_device_auto. The total run-time memory
used is mostly the same, we reduce the code size a little, and as a
bonus, struct serial_dev_priv does not contain the unused members when
!SERIAL_RX_BUFFER.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial-uclass.c

@@ -588,11 +588,6 @@ static int serial_post_probe(struct udevice *dev)
 	sdev.getc = serial_stub_getc;
 	sdev.tstc = serial_stub_tstc;
 
-#if CONFIG_IS_ENABLED(SERIAL_RX_BUFFER)
-	/* Allocate the RX buffer */
-	upriv->buf = malloc(CONFIG_SERIAL_RX_BUFFER_SIZE);
-#endif
-
 	stdio_register_dev(&sdev, &upriv->sdev);
 #endif
 	return 0;