hwpstate_amd(4): Rename register dump sysctl handler

[olce]

Summary

While here, rename the callback function and structure to a more
immediately informative name.

While here, remove a superfluous cast and rename the variable containing
the callback data to just 'data' instead of 'req' which can be confusing
(because of CPPC_REQUEST but also the fact that 'req' is a parameter of
sysctl handlers).

No functional change intended.

Sponsored by: The FreeBSD Foundation