net/at91_ether: use gpio_to_irq for phy IRQ line

Use the gpio_to_irq() function to retrieve the phy IRQ line
from the GPIO pin specification.
This fix is needed now that we have moved to irqdomains on AT91.

Reported-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Andrew Victor <avictor.za@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>