serial: 8250_dw: Take port lock while accessing LSR

Accessing LSR requires port lock because it mutates lsr_saved_flags
in serial_lsr_in().

Fixes: 197eb5c4 ("serial: 8250_dw: Use serial_lsr_in() in dw8250_handle_irq()")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/c5879db7-bee9-93f-526e-872a292442@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dw.c

@@ -266,7 +266,10 @@ static int dw8250_handle_irq(struct uart_port *p)
 
 	/* Manually stop the Rx DMA transfer when acting as flow controller */
 	if (quirks & DW_UART_QUIRK_IS_DMA_FC && up->dma && up->dma->rx_running && rx_timeout) {
+		spin_lock_irqsave(&p->lock, flags);
 		status = serial_lsr_in(up);
+		spin_unlock_irqrestore(&p->lock, flags);
+
 		if (status & (UART_LSR_DR | UART_LSR_BI)) {
 			dw8250_writel_ext(p, RZN1_UART_RDMACR, 0);
 			dw8250_writel_ext(p, DW_UART_DMASA, 1);