net: e1000: implement eth_write_hwaddr

net: e1000: implement eth_write_hwaddr

Implement programming MAC address to the hardware, i.e. external flash
seen as EEPROM.

MAC address is only written if it differs from what is already stored in
flash or if reading the current MAC address fails.

Signed-off-by: Hannu Lounento <hannu.lounento@ge.com>
CC: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>