net: dsa: Initialize ds->cpu_port_mask earlier

The mt7530 driver has its dsa_switch_ops::get_tag_protocol function
check ds->cpu_port_mask to issue a warning in case the configured CPU
port is not capable of supporting tags.

After commit 14be36c2 ("net: dsa: Initialize all CPU and enabled
ports masks in dsa_ds_parse()") we slightly re-arranged the
initialization such that this was no longer working. Just make sure that
ds->cpu_port_mask is set prior to the first call to get_tag_protocol,
thus restoring the expected contract. In case of error, the CPU port bit
is cleared.

Fixes: 14be36c2 ("net: dsa: Initialize all CPU and enabled ports masks in dsa_ds_parse()")
Reported-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>