serial: sh-sci: Fix up break timer scheduling race.

The break flag timer is presently added through add_timer() via the
interrupt and error paths, where it is possible to send multiple breaks
in rapid succession and trigger the timer pending BUG_ON(). This moves
over to a mod_timer() instead.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

drivers/serial/sh-sci.c

@@ -554,8 +554,7 @@ static void sci_receive_chars(struct uart_port *port)
  */
 static inline void sci_schedule_break_timer(struct sci_port *port)
 {
-	port->break_timer.expires = jiffies + SCI_BREAK_JIFFIES;
-	add_timer(&port->break_timer);
+	mod_timer(&port->break_timer, jiffies + SCI_BREAK_JIFFIES);
 }
 
 /* Ensure that two consecutive samples find the break over. */