net: macb: Remove redundant poll irq assignment

In phy_device's general probe, this device will already be set for
phy register polling, rendering this code redundant.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>