net: ibm: emac: replace custom rgmii_mode_name with phy_modes

phy_modes() in the common phy.h already defines the same phy mode
names in lower case. The deleted rgmii_mode_name() is used only
in one place and for a "notice-level" printk. Hence, it will not
be missed.

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