r8169: add helper rtl_pci_commit

In few places we do a PCI commit by reading an arbitrary chip register.
It's not always obvious that the read is meant to be a PCI commit,
therefore add a helper for it.

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