PCI: Return NULL for to_pci_driver(NULL)

to_pci_driver() takes a pointer to a struct device_driver and uses
container_of() to find the struct pci_driver that contains it.

If given a NULL pointer to a struct device_driver, return a NULL pci_driver
pointer instead of applying container_of() to NULL.

This simplifies callers that would otherwise have to check for a NULL
pointer first.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>