Exception due to PCI: Use pci_is_root_bus() to check for root bus

[David Engraf <david.engraf@sysgo.com>]

Hi,

I have an exception in __pci_bus_size_bridges() when pci_is_root_bus 
returns false but bus->self == NULL. My driver registers a virtual bus, 
like virtfn_add_bus(). pci_add_new_bus() is called with a parent but 
without a pci_dev. Thus bus->parent is set but bus->self is NULL. When 
__pci_bus_size_bridges() is called I get an exception at:

switch (bus->self->class >> 8)

The previous version of the code, checking for bus->self != NULL worked 
for me. I think an additional check is required to make sure we're not 
accessing a NULL pointer.

Best regards
David Engraf