net: dsa: add ds and index to dsa_port

Add the physical switch instance and port index a DSA port belongs to to
the dsa_port structure.

That can be used later to retrieve information about a physical port
when configuring a switch fabric, or lighten up struct dsa_slave_priv.

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