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>