staging: fsl-dpaa2/eth: Fix skb use after free

Once a Tx frame descriptor is enqueued, an interrupt might be triggered
to process the Tx confirmation and free the skb, hitting a memory use
after free when updating the tx_bytes statistic based on skb->len.

Use the frame descriptor length instead.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>