i40e: Use LLDP ethertype define ETH_P_LLDP

Remove references to I40E_ETH_P_LLDP and use ETH_P_LLDP instead.

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>