ARM: shmobile: kzm9g-reference: Use falling edge IRQ for LAN9221

The device is configured to generate an active-low interrupt signal that
is automatically deasserted without requiring any action from the host.
Configure the IRQ to trigger on falling edge.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>