net: ethernet: sxgbe: remove private tx queue lock

The driver uses a private lock for synchronization of the xmit function and
the xmit completion handler, but since the NETIF_F_LLTX flag is not set,
the xmit function is also called with the xmit_lock held.

On the other hand the completion handler uses the reverse locking order by
first taking the private lock and (in case that the tx queue had been
stopped) then the xmit_lock.

Improve the locking by removing the private lock and using only the
xmit_lock for synchronization instead.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>