Question about uart_xmitchars()

Hi, i was running nuttx flat build on LoongArch 3A5000 CPU(not qemu). I\`ve ported this version earlier.
It\`s strange that `NuttShell (NSH) NuttX-12.0.0` could not be printed from uart. So i checked the code and found out that line 65 will judge whether the uart_txready is true.
https://github.com/apache/nuttx/blob/a720984eb7b1cabd259014df0d4e9ac0ef0e94dd/drivers/serial/serial_io.c#L65

If it\`s false, uart just won\`t print anything.

Shall we change the code to synchronously wait for uart_txready? Like this:

```
// while (dev->xmit.head != dev->xmit.tail && uart_txready(dev))
while (dev->xmit.head != dev->xmit.tail)
{
/* Send the next byte */

uart_send(dev, dev->xmit.buffer[dev->xmit.tail]);//Call u16550_send function below
nbytes++;

/* Increment the tail index */

if (++(dev->xmit.tail) >= dev->xmit.size)
{
dev->xmit.tail = 0;
}
}
```

In u16550_send change it like this:
```
static void u16550_send(struct uart_dev_s *dev, int ch)
{
while (!uart_txready(dev)); //wait untill it`s ready
FAR struct u16550_s *priv = (FAR struct u16550_s *)dev->priv;
u16550_serialout(priv, UART_THR_OFFSET, (uart_datawidth_t)ch);
}
```

I\`ve also added this in u16550_setup() so that showprogress could print out ABC successfully.
```
static int u16550_setup(FAR struct uart_dev_s *dev)
{
//other codes
/* Clear fifos */
while ((u16550_serialin(priv, UART_LSR_OFFSET) & UART_LSR_THRE) == 0);//newly added
u16550_serialout(priv, UART_FCR_OFFSET,
(UART_FCR_RXRST | UART_FCR_TXRST));
```