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>