arch: arm64: Fix qemu_pl011_txint() in qemu_serial.c

Summary:
- I noticed that the nsh prompt can not be shown when disabling
  debug features. Actually, the prompt will be shown when a user
  input happens.
- This commit fixes this issue by adding uart_xmitchars() as
  other serial drivers do.

Impact:
- None

Testing:
- Tested with qemu-armv8a:netnsh on QEMU-7.1

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

arch/arm64/src/qemu/qemu_serial.c

@@ -458,15 +458,26 @@ static void qemu_pl011_rxint(struct uart_dev_s *dev, bool enable)
 static void qemu_pl011_txint(struct uart_dev_s *dev, bool enable)
 {
   struct pl011_uart_port_s *sport = (struct pl011_uart_port_s *)dev->priv;
+  irqstate_t flags;
+
+  flags = enter_critical_section();
 
   if (enable)
     {
       pl011_irq_tx_enable(sport);
+
+      /* Fake a TX interrupt here by just calling uart_xmitchars() with
+       * interrupts disabled (note this may recurse).
+       */
+
+      uart_xmitchars(dev);
     }
   else
     {
       pl011_irq_tx_disable(sport);
     }
+
+  leave_critical_section(flags);
 }
 
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