PCI: Add pci_wait_for_pending_transaction()

New routine to avoid duplication of code to wait for pending PCI
transactions to complete.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>