net: dsa: mv88e6xxx: allocate the number of ports

The current code allocates DSA_MAX_PORTS ports for a Marvell dsa_switch
structure. Provide the exact number of ports so the corresponding
ds->num_ports is accurate.

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>