pci: Export bus numbers for bridge devices in struct pci_conf

This exports bus information about bridges to userspace via the
less-privileged PCIOCGETCONF ioctl. Previously if userspace wished to
query this information, it had to use direct PCI config register
access which requires higher privilege.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D55771