hw.bus.info: rework handler

hw.bus.info was added in r68522 as a node, but there was never anything
connected "behind" it. Its only purpose is to return a struct u_businfo.
The only in-base consumer are devinfo(3)/devinfo(8).
Rewrite the handler as SYSCTL_PROC and mark it as MPSAFE and read-only
as there never was a writable path.

Reviewed by: kib
Approved by: kib (mentor)
Sponsored by: Mysterious Code Ltd.
Differential Revision: https://reviews.freebsd.org/D25321