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

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: c25b23b8a387 ("bgmac: register fixed PHY for ARM BCM470X / BCM5301X chipsets")
Signed-off-by: Ruan Jinjie <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Reviewed-by: Leon Romanovsky <[email protected]>
Signed-off-by: David S. Miller <[email protected]>