PCI: fix upstream P2P bridge checks when enabling OBFF and LTR

PCI: fix upstream P2P bridge checks when enabling OBFF and LTR

pci_enable_obff() and pci_enable_ltr() incorrectly check "dev->bus" instead
of "dev->bus->self" to determine whether the upstream device is a P2P
bridge or a host bridge.  For devices on the root bus, the upstream device
is a host bridge, "dev->bus != NULL" and "dev->bus->self == NULL", and we
panic with a null pointer dereference.

These functions should previously have panicked when called on devices
supporting OBFF or LTR, so they should be regarded as untested.

Found by Coverity (CID 143038 and CID 143039).

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