Store a pointer to the dsa_port structure in the dsa_slave_priv
structure, instead of the switch/port index.

This will allow to store more information such as the bridge device,
needed in DSA drivers for multi-chip configuration.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>