net: greth: Utilize of_get_mac_address()

Do not open code getting the MAC address exclusively from the
"local-mac-address" property, but instead use of_get_mac_address() which
looks up the MAC address using the 3 typical property names.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>