net: dsa: add dsa_to_port helper

The dsa_port structure is part of DSA core data and must only be updated
by the later. It is OK and sometimes necessary for the DSA drivers to
access this data, but this has to be read only.

For that purpose, add a dsa_to_port() helper which returns a const
pointer to a dsa_port structure which must be used by DSA drivers from
now on instead of digging into ds->ports[] themselves.

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