net/at91_ether: use gpio_is_valid for phy IRQ line

Use the generic gpiolib gpio_is_valid() function to test
if the phy IRQ line GPIO is actually provided.

For non-connected or non-existing phy IRQ lines, -EINVAL
value is used for phy_irq_pin field of struct at91_eth_data.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: David S. Miller <davem@davemloft.net>