serial: meson: fix meson_serial_pending() tx logic

According to the dm_serial_ops documentation, pending() should:
> @return number of waiting characters, 0 for none, -ve on error

And:
> It is acceptable to return 1 if an indeterminant number
> of characters is waiting.

With the current implementation, we have:
* FIFO is full           -> pending() returns 0
* FIFO is partially used -> pending() returns 1
* FIFO is empty          -> pending() returns 1

This is not the same as what the documentation requires.

Moreover, since [1], arm reset now flushes all console devices
(including serial) before the cpu gets reset.
Because of the flawed logic:

  => reset # user calls reset
  flush() is called
  _serial_flush() is called
  ops->pending(dev, false) # never returns false
  # board hangs indefinitely without resetting.

Fix it by using AML_UART_TX_EMPTY instead of AML_UART_TX_FULL.

[1] commit c5f4cdb8eb60 ("console: Use flush() before panic and reset"),

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230606-fix-meson-serial-pending-v1-1-6a54d4a01f76@baylibre.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>

drivers/serial/serial_meson.c

@@ -201,7 +201,10 @@ static int meson_serial_pending(struct udevice *dev, bool input)
 
 		return true;
 	} else {
-		return !(status & AML_UART_TX_FULL);
+		if (status & AML_UART_TX_EMPTY)
+			return false;
+
+		return true;
 	}
 }