net: dsa: Initialize ds->enabled_port_mask and ds->phys_mii_mask

Some drivers rely on these two bitmasks to contain the correct values
for them to successfully probe and initialize at drv->setup() time,
calculate correct values to put in both masks as early as possible in
dsa_get_ports_dn().

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>