net: WIZnet drivers: fix possible NULL dereference

This fixes possible null dereference in probe() function: when both
.mac_addr and .link_gpio are unknown, dev.platform_data may be NULL

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mike Sinkovsky <msink@permonline.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>