net: bgmac: 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: c25b23b8 ("bgmac: register fixed PHY for ARM BCM470X / BCM5301X chipsets")
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>