net: Add a define for LLDP ethertype

Add a new define ETH_P_LLDP for Link Layer Discovery Protocol (LLDP)
ethertype.

Suggested-by: Bruce Allan <bruce.w.allan@intel.com>
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>