net: mvmdio: avoid error message for optional IRQ

Per the dt-binding the interrupt is optional so use
platform_get_irq_optional() instead of platform_get_irq(). Since
commit 7723f4c5 ("driver core: platform: Add an error message to
platform_get_irq*()") platform_get_irq() produces an error message

  orion-mdio f1072004.mdio: IRQ index 0 not found

which is perfectly normal if one hasn't specified the optional property
in the device tree.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvmdio.c

@@ -347,7 +347,7 @@ static int orion_mdio_probe(struct platform_device *pdev)
 	}
 
 
-	dev->err_interrupt = platform_get_irq(pdev, 0);
+	dev->err_interrupt = platform_get_irq_optional(pdev, 0);
 	if (dev->err_interrupt > 0 &&
 	    resource_size(r) < MVMDIO_ERR_INT_MASK + 4) {
 		dev_err(&pdev->dev,