serial: sh-sci: Return IRQ_HANDLED when overrun if detected

This patch fix an issue that the driver may cause "nobody cared" IRQ
when this driver detects the overrun flag only.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1078,8 +1078,10 @@ static irqreturn_t sci_mpxed_interrupt(int irq, void *ptr)
 		ret = sci_br_interrupt(irq, ptr);
 
 	/* Overrun Interrupt */
-	if (orer_status & s->overrun_mask)
+	if (orer_status & s->overrun_mask) {
 		sci_handle_fifo_overrun(port);
+		ret = IRQ_HANDLED;
+	}
 
 	return ret;
 }