r8169: remove unneeded mmiowb barriers

writex() has implicit barriers, that's what makes it different from
writex_relaxed(). Therefore these calls to mmiowb() can be removed.

This patch was recently reverted due to a dependency with another
problematic patch. But because it didn't contribute to the problem
it was rebased and can be resubmitted.

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