irqchip: orion: Fix getting generic chip pointer.

Enabling SPARSE_IRQ shows up a bug in the irq-orion bridge interrupt
handler. The bridge interrupt is implemented using a single generic
chip. Thus the parameter passed to irq_get_domain_generic_chip()
should always be zero.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Fixes: 9dbd90f1 ("irqchip: Add support for Marvell Orion SoCs")
Cc: <stable@vger.kernel.org> # v3.11+
Signed-off-by: Jason Cooper <jason@lakedaemon.net>