net: systemport: fix potential memory leak in bcm_sysport_xmit()
In the Linux kernel, the following vulnerability has been resolved:

net: systemport: fix potential memory leak in bcm_sysport_xmit()

The bcm_sysport_xmit() returns NETDEV_TX_OK without freeing skb
in case of dma_map_single() fails, add dev_kfree_skb() to fix it.