atmel_serial: fix interrupt handler return value

We should only return IRQ_HANDLED when we actually found something to
handle. This is important since the USART interrupt handler may be
shared with the timer interrupt on some chips.

Pointed-out-by: michael <trimarchi@gandalf.sssup.it>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/atmel_serial.c

@@ -549,7 +549,7 @@ static irqreturn_t atmel_interrupt(int irq, void *dev_id)
 		atmel_handle_transmit(port, pending);
 	} while (pass_counter++ < ATMEL_ISR_PASS_LIMIT);
 
-	return IRQ_HANDLED;
+	return pass_counter ? IRQ_HANDLED : IRQ_NONE;
 }
 
 /*