net: systemport: Unmap queues upon DSA unregister event

Binding and unbinding the switch driver which creates the DSA slave
network devices for which we set-up inspection would lead to
undesireable effects since we were not clearing the port/queue mapping
to the SYSTEMPORT TX queue.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>