net: dsa: mv88e6xxx: add number of ports to info

Drop the ps->num_ports variable in favor of a new member of the info
structure. This removes the need to assign it at setup time.

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>