pch_gbe: replace private tx ring lock with common netif_tx_lock

pch_gbe_tx_ring.tx_lock is only used in the hard_xmit handler and
in the transmit completion reaper called from NAPI context.

Compile-tested only. Potential victims Cced.

Someone more knowledgeable may check if pch_gbe_tx_queue could
have some use for a mmiowb.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Andy Cress <andy.cress@us.kontron.com>
Cc: bryan@fossetcon.org
Signed-off-by: David S. Miller <davem@davemloft.net>