pcib: Make various bus methods private to pci_pci.c

These functions are not used by pcib subclasses so do not need to
be exposed in pcib_private.h.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43688

(cherry picked from commit 03719c651430d46cd8aa95aead02a7f0c43dcb04)