net: dsa: store CPU switch structure in the tree

Store a dsa_switch pointer to the CPU switch in the tree instead of only
its index. This avoids the need to initialize it to -1.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>