Documentation: networking: dsa: Remove priv_size description

We no longer have a priv_size structure member since 5feebd0a ("net:
dsa: Remove allocation of driver private memory")

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