net: dsa: use ds->num_ports when possible

The dsa_switch structure contains the number of ports. Use it where the
structure is valid instead of the DSA_MAX_PORTS value.

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