i40e: use eth_platform_get_mac_address()

This commit converts commit b499ffb0 ("i40e: Look up MAC address in
Open Firmware or IDPROM") to use eth_platform_get_mac_address()
added by commit c7f5d105

 ("net: Add eth_platform_get_mac_address()
helper.")

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>