Revert "r8169: remove unneeded mmiowb barriers"

This reverts commit bd7153bd

.

There doesn't seem to be anything wrong with this patch,
it's just reverted to get a stable baseline again.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>