PCI: hotplug: Remove unnecessary "dev->bus" test

Every pci_dev is on a valid pci_bus, so we don't need to test whether
dev->bus is NULL or not.

The only exceptions are a few legacy cases like alpha_core_agp_setup(),
parisc_agp_setup(), and megaraid's make_local_pdev(), where we allocate a
pci_dev with a NULL bus pointer.  These are dubious uses (especially the
megaraid one), and I don't think it's possible to exercise this
pci_configure_slot() path with any of them.

Found by Coverity (CID 146446).

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