net: dsa: mv88e6xxx: replace ds with ps where possible

The dsa_switch structure ds is actually needed in very few places,
mostly during setup of the switch. The private structure ps is however
needed nearly everywhere. Pass ps, not ds internally.

[vd: rebased Andrew's patch.]

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