net: systemport: do not crash freeing an unitialized TX ring

Callers of bcm_sysport_init_tx_ring() can currently fail, and will
always call bcm_sysport_fini_tx_ring() in a loop ending at the number of
TX queues (32) without checking if the TX ring was successfully
initialized or not.

Update bcm_sysport_fini_tx_ring() to return early and avoid a crash
de-referencing ring->cbs if the TX ring was not initialized, since
ring->cbs is the last part of the initialization done by
bcm_sysport_init_tx_ring() that could fail.

Fixes: 80105bef

 ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>