net: e1000: add support for writing to EEPROM

Port functions for writing to EEPROM, updating the checksum and
committing data to flash from the Linux kernel igb driver.

Functions were ported from Linux 4.8-rc2 (694d0d0bb20).

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>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>