net: bgmac: Return PTR_ERR() for fixed_phy_register()

fixed_phy_register() returns -EPROBE_DEFER, -EINVAL and -EBUSY,
etc, in addition to -ENODEV. The best practice is to return
these error codes with PTR_ERR().

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>