Fix ofw pcib when it rman_init fails

rman_fini assumes rman_init has been called successfully. Clean up
init faulure by only calling rman_fini when rman_init has succeeded.

While here add ofw_pcib_fini that can be used by a sub-class to clean
up.

Reviewed by: bz, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35681