net: ucc_geth: Don't use the MAC as PHY without a fixed link

This matches what the other drivers using fixed-link support do and
restores the behaviour before commit 87009814 ("ucc_geth: use the
new fixed PHY helpers") for the affected device trees (i.e. no
phy-handle and no fixed-link).

Fixes: 87009814 ("ucc_geth: use the new fixed PHY helpers")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>