net: ibm: emac: replace custom PHY_MODE_* macros

The ibm_emac driver predates the PHY_INTERFACE_MODE_*
enums by a few years.

And while the driver has been retrofitted to use the PHYLIB,
the old definitions have stuck around to this day.

This patch replaces all occurences of PHY_MODE_* with
the respective equivalent PHY_INTERFACE_MODE_* enum.
And finally, it purges the old macros for good.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>