net: broadcom: bcm4908_enet: update TX stats after actual transmission

In the Linux kernel, the following vulnerability has been resolved: net: broadcom: bcm4908_enet: update TX stats after actual transmission Queueing packets doesn't guarantee their transmission. Update TX stats after hardware confirms consuming submitted data. This also fixes a possible race and NULL dereference. bcm4908_enet_start_xmit() could try to access skb after freeing it in the bcm4908_enet_poll_tx().