ethernet: use likely() for common Ethernet encap

Mark code path's likely/unlikely based on most common usage.
  * Very few devices use dsa tags.
  * Most traffic is Ethernet (not 802.2)
  * No sane person uses trailer type or Novell encapsulation

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>