hwpstate_amd(4): Rename EPP handler, convert to unsigned int

'val' is supposed to be the storage for a sysctl knob of an 'unsigned
int', so consistently don't specify its width.

While here, rename variables in the handler. The new names feel
clearer, and the 'ret' => 'error' rename is to be more in line with our
tradition.

No functional change intended.

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55002