net: bcmgenet: Fix return value check for fixed_phy_register()

The fixed_phy_register() function returns error pointers and never
returns NULL. Update the checks accordingly.

Fixes: b0ba512e

 ("net: bcmgenet: enable driver to work without a device tree")
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>