net: e1000: split e1000_read_mac_addr

net: e1000: split e1000_read_mac_addr

Split the implementation of e1000_read_mac_addr into eeprom and register
versions called by e1000_read_mac_addr.

This allows for calling e1000_read_mac_addr when MAC address is needed
with no constraints where it is read from, and for calling the register
and, especially, the eeprom version directly in order to specify where
to read the address from.

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>