net: ucc_geth: make probe consistently acquire a reference to the phy node

When the driver attaches to a device that has a phy handle the probe
routine returns with a reference to that node. This reference is
correctly dropped in the error path and the remove function. In the
fixed phy case however no reference is acquired and so the error path
might drop a reference the driver isn't holding. Fix that by getting a
reference to the MAC.

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>