r8169: consider that 32 Bit DMA is the default

Documentation/DMA-API-HOWTO.txt states:
By default, the kernel assumes that your device can address 32-bits of
DMA addressing. For a 64-bit capable device, this needs to be increased,
and for a device with limitations, it needs to be decreased.

Therefore we don't need the 32 Bit DMA fallback configuration and can
remove it.

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