hwpmc_mod.c: misc style cleanup

Everything else I found worthy of adjustment.

• Order local variable declarations per style(9)
• Make use of __unused annotations rather than cast to void
• Remove unnecessary casts
• Add (void) casts to PMC class methods where the return value is ignored
• A couple instances of reordering statements for clarity
• Prefer bool type where applicable
• unsigned int -> u_int
• Use uintmax_t/%j in printf calls
• Formatting of comments

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40517