net: systemport: Dynamically allocate number of TX rings

In preparation for adding SYSTEMPORT Lite, which has twice as less transmit
queues than SYSTEMPORT make sure we do allocate TX rings based on the
systemport,txq property to get an appropriate memory footprint.

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