802.3ad: use standard ethhdr instead of ad_header

802.3ad has its own ethhdr-like structure in the form of an ad_header,
which is at the start of both the LACPDU and marker PDU.  Both are
the same from the struct values, both are packed as well.

It's therefore perfectly fine to replace the ad_header by the ethhdr
and to remove its definition.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: David S. Miller <davem@davemloft.net>