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.

Fixes: 80105bef ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Link: https://patch.msgid.link/20241014145115.44977-1-wanghai38@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>