r8169: improve pci region handling

The region to be used is always the first of type IORESOURCE_MEM.
We can implement this rule directly w/o having to specify which
region is the first one per configuration entry.

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