ixgbe: Look up MAC address in Open Firmware or IDPROM

Attempt to look up the MAC address in Open Firmware on systems that
support it. On SPARC resort to using the IDPROM if no OF address is
found.

Signed-off-by: Martin K Petersen <martin.petersen@oracle.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>