net: axienet: 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.

Also avoid casting away the const qualifier of the return value by
making axienet_set_mac_address() take a const void* address.

Follows commit b34296a9 ("net: ethoc: Utilize
of_get_mac_address()").

Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>