e1000e: use alternate MAC address on ESB2 if available

Similar to 82571/2/3 parts that already do this, if ESB2/80003es2lan parts
have an alternate MAC address provided in the EEPROM use it instead of the
default MAC address.  This patch makes the the actual code that does this
generic so that it can be better used by both MAC families.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>