imx9/lpuart: Fix SMP deadlock with imx9_txint

A deadlock occurs for priv->lock when uart_xmitchars is called from
within imx9_txint. This is because uart_xmitchars does a callback to
imx9_txint and tries to acquire the lock again.

However, there is no reason to hold the lock over uart_xmitchars, as it
has its own lock for mutual exclusion. Only the UART device needs to
be protected, and priv->lock does that.

Signed-off-by: Ville Juven <ville.juven@unikie.com>

arch/arm64/src/imx9/imx9_lpuart.c

@@ -2358,6 +2358,7 @@ static void imx9_txint(struct uart_dev_s *dev, bool enable)
   regval &= ~LPUART_ALL_INTS;
   regval |= priv->ie;
   imx9_serialout(priv, IMX9_LPUART_CTRL_OFFSET, regval);
+  spin_unlock_irqrestore(&priv->lock, flags);
 
 #ifndef CONFIG_SUPPRESS_SERIAL_INTS
   if (enable)
@@ -2365,8 +2366,6 @@ static void imx9_txint(struct uart_dev_s *dev, bool enable)
       uart_xmitchars(dev);
     }
 #endif
-
-  spin_unlock_irqrestore(&priv->lock, flags);
 }
 #endif