net: systemport: Only inspect valid switch port & queues

Hesoteric board configurations where port 0 is not available would still
make SYSTEMPORT inspect the switch port 0, queue 0, which, not being
enabled, would cause transmit timeouts over time. Just ignore those
unconfigured rings instead.

Fixes: 84ff33eeb23d ("net: systemport: Establish DSA network device queue mapping")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>