r8169: remove PCI DAC support

The old non-PCIe chip versions support PCI DAC, however this feature
seems to be fragile, see comment in the PCI error handler. Therefore
it's disabled per default. I think meanwhile it's time remove support
for this legacy feature. This helps to reduce complexity of the driver.

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