ks8851: companion eeprom access through ethtool

Accessing ks8851 companion eeprom permits modifying the ks8851 stored
MAC address.

Example how to change the MAC address using ethtool, to set the
01:23:45:67:89:AB MAC address:
$ echo "0:AB8976452301" | xxd -r > mac.bin
$ sudo ethtool -E eth0 magic 0x8870 offset 2 < mac.bin

Signed-off-by: Sebastien Jan <s-jan@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>