serial: 8250-fsl: Only do the break workaround if IIR signals RLSI

It can happen that while a break is received the transmitter gets empty
and IIR signals a Transmitter holding register empty (THRI) event. In
this case it's too early for the break workaround. Still doing it then
results in the THRI event not being rereported which makes the driver
miss that and e.g. for RS485 half-duplex communication it fails to
switch back to RX mode.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20231213174312.2341013-1-u.kleine-koenig@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_fsl.c

@@ -51,7 +51,8 @@ int fsl8250_handle_irq(struct uart_port *port)
 	 * immediately and interrupt the CPU again. The hardware clears LSR.BI
 	 * when the next valid char is read.)
 	 */
-	if (unlikely(up->lsr_saved_flags & UART_LSR_BI)) {
+	if (unlikely((iir & UART_IIR_ID) == UART_IIR_RLSI &&
+		     (up->lsr_saved_flags & UART_LSR_BI))) {
 		up->lsr_saved_flags &= ~UART_LSR_BI;
 		port->serial_in(port, UART_RX);
 		uart_port_unlock_irqrestore(&up->port, flags);