serial: altera: don't enable any irq if the device doesn't feature an irq

If the irq line of an altera UART device isn't used to report interrupts
for this device the driver better ensures that this device doesn't pull
this line to active state and so disturb the whatever might be connected
to this line.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>