net: systemport: Guard against unmapped TX ring

Because SYSTEMPORT is a (semi) normal network device, the stack may attempt to
queue packets on it oustide of the DSA slave transmit path.  When that happens,
the DSA layer has not had a chance to tag packets with the appropriate per-port
and per-queue information, and if that happens and we don't have a port 0 queue
0 available (e.g: on boards where this does not exist), we will hit a NULL
pointer de-reference in bcm_sysport_select_queue().

Guard against such cases by testing for the TX ring validity.

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>