net: bcmgenet: Fix NULL vs IS_ERR() checking

The phy_attach() function does not return NULL. It returns error pointers.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>