serial: lantiq: add missing interrupt ack

Currently, the error interrupt is never acknowledged, so once active it
will stay active indefinitely, causing the handler to be called in an
infinite loop.

Fixes: 2f0fc41 ("SERIAL: Lantiq: Add driver for MIPS Lantiq SOCs.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Bernhard Seibold <mail@bernhard-seibold.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Message-ID: <20230602133029.546-1-mail@bernhard-seibold.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: bseibold

drivers/tty/serial/lantiq.c

@@ -250,6 +250,7 @@ lqasc_err_int(int irq, void *_port)
 	struct ltq_uart_port *ltq_port = to_ltq_uart_port(port);
 
 	spin_lock_irqsave(&ltq_port->lock, flags);
+	__raw_writel(ASC_IRNCR_EIR, port->membase + LTQ_ASC_IRNCR);
 	/* clear any pending interrupts */
 	asc_update_bits(0, ASCWHBSTATE_CLRPE | ASCWHBSTATE_CLRFE |
 		ASCWHBSTATE_CLRROE, port->membase + LTQ_ASC_WHBSTATE);