net: dsa: Initialize all CPU and enabled ports masks in dsa_ds_parse()

There was no reason for duplicating the code that initializes
ds->enabled_port_mask in both dsa_parse_ports_dn() and
dsa_parse_ports(), instead move this to dsa_ds_parse() which is early
enough before ops->setup() has run.

While at it, we can now make dsa_is_cpu_port() check ds->cpu_port_mask
which is a step towards being multi-CPU port capable.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>