net: systemport: fix bcm_sysport_insert_tsb()

Similar to commit bc23333b ("net:
bcmgenet: fix bcmgenet_put_tx_csum()"), we need to return the skb
pointer in case we had to reallocate the SKB headroom.

Fixes: 80105bef ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>