hw.bus.info: rework handler

[kaktus]

Summary

hw.bus.info was added in rS68522 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 is 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.

[kib]

I assume you checked that devinfo(8) still works.