hwpmc_mod.c: whitespace style cleanup

Handle a few things related to spacing:

• Remove redundant/superfluous blank lines (and add a couple where helpful)
• Add spacing around binary operators
• Remove spacing after casts and before goto labels
• Adjustments for line width of 80 chars
• Tab/space character issues

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40514