i40e: allow look-up of MAC address from Open Firmware or IDPROM

Look up the MAC address from the eth_get_platform_mac_address() function
first before checking what the firmware provides. We already handle the
case of re-writing the MAC-VLAN filter, so there is no need to add extra
code for this. However, update the comment where we do this to indicate
that it does impact the Open Firmware MAC address case.

Change-ID: I73e59fbe0b0e7e6f3ee9f5170d0bd3a4d5faf4db
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>