bge: check that the bus is a pci bus before using it as such

This fixes the following panic on powerpc:

pci_get_vendor failed for pcib1 on bus ofwbus0, error = 2

PR: 238730
Reported by: Dennis Clarke <dclarke@blastwave.org>
Tested by: Dennis Clarke <dclarke@blastwave.org>
MFC after: 2 weeks