dsa: Move all definitions needed by drivers into <net/dsa.h>

Any headers included by drivers should be under include/, and
any definitions they use are not really private to the core as
the name "dsa_priv.h" suggests.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>