serial: imx: let irq handler return IRQ_NONE if no event was handled

This gives the irq core a chance to disable the serial interrupt in case
an event isn't cleared in the handler.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>