hwpstate_amd(4): Rename register dump sysctl handler

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.

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55000