net: dsa: include switchdev.h only once

DSA drivers and core use switchdev. Include switchdev.h only once, in
the dsa.h public header, so that inclusion in DSA drivers or forward
declarations of switchdev structures in not necessary anymore.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>