hwpmc_mod.c: conditional and loop style

• Explicitly check the value in conditional statements, per style(9)
• Add braces around more loops and conditionals, wherever it appears (to me) to aid legibility
• Expand some assignments within if statements
• Fix formatting around list-type FOREACH macros

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40515