net: dsa: add a generic procedure for the flow dissector

For all DSA formats that don't use tail tags, it looks like behind the
obscure number crunching they're all doing the same thing: locating the
real EtherType behind the DSA tag. Nonetheless, this is not immediately
obvious, so create a generic helper for those DSA taggers that put the
header before the EtherType.

Another assumption for the generic function is that the DSA tags are of
equal length on RX and on TX. Prior to the previous patch, this was not
true for ocelot and for gswip. The problem was resolved for ocelot, but
for gswip it still remains, so that can't use this helper yet.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>