Unfortunately, our current implementation of 'wmb()' doesn't seem to do what we
want it to do. On base-hw + imx6q_sabrelite, the write of bdp->cbd_sc seems to
get re-ordered after the write to txq->bd.reg_desc_active in the transmission
path of the contrib code. Due to this, the transmission of the packet is only
triggered the next time a packet is sent. However, we only quick-fix it by
enforcing the execution of the write with a volatile global read as we will
soon update the FEC NIC port with a new DDE approach anyway.
Fixes#4010
ff128df131