net: ethernet: ti: cpsw: fix fixed-link phy probe deferral

Make sure to propagate errors from of_phy_register_fixed_link() which
can fail with -EPROBE_DEFER.

Fixes: 1f71e8c9 ("drivers: net: cpsw: Add support for fixed-link
PHY")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>