totemknet: Remove usage of struct ether_header

Location of struct ether_header is not very well defined so implement
alternative approach of not use struct ether_header at all.

We know nozzle packet is Ethernet II format, which is strictly defined,
equal across OSes/architectures and never changes so we can just use
location of destination MAC address, what is first 6 bytes (octets) of
the packet.

Also check of validity of packet is added.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>