serial: 8250: Remove low_latency workaround

The defunct low_latency input steering executed flush_to_ldisc()
directly from interrupt context so dropping the port lock was
necessary to avoid deadlock. That steering was removed by

    commit a9c3f68f
    Author: Peter Hurley <peter@hurleysoftware.com>
    Date:   Sat Feb 22 07:31:21 2014 -0500

    tty: Fix low_latency BUG
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -1489,9 +1489,8 @@ unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
 			break;
 		lsr = serial_in(up, UART_LSR);
 	} while (lsr & (UART_LSR_DR | UART_LSR_BI));
-	spin_unlock(&port->lock);
+
 	tty_flip_buffer_push(&port->state->port);
-	spin_lock(&port->lock);
 	return lsr;
 }
 EXPORT_SYMBOL_GPL(serial8250_rx_chars);