net: dsa: mv88e6xxx: drop double ds assignment

Every driver assigns ps->ds even though it gets assigned in the shared
mv88e6xxx_setup_common function. Kill redundancy.

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