PCI: Add pcim_set_mwi(), a device-managed pci_set_mwi()

Add pcim_set_mwi(), a device-managed version of pci_set_mwi().
First user is the Realtek r8169 driver.

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