PCI: Add pci_status_get_and_clear_errors

Several drivers use the following code sequence:
1. Read PCI_STATUS
2. Mask out non-error bits
3. Action based on error bits set
4. Write back set error bits to clear them

As this is a repeated pattern, add a helper to the PCI core.

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