hwpstate_amd(4): Register dump: Fine-grained error reporting

[olce]

Summary

If some of the registers cannot be read, report that but continue trying
reading the others. This also has the side benefit of simplifying code.

While here, use sbuf_new_for_sysctl(), and rename 'res' and 'ret', which
are to contain error values, to 'error'.

While here, remove the test on getting the per-cpu structure, as if it
is not present we would have already crashed on device attach.

PR: 292615
Sponsored by: The FreeBSD Foundation