hwpmc: split out PMC_OP_PMCRW

Split out the functional logic from the syscall handler into a helper
function. This keeps it separate from the syscall control-flow logic,
resulting in better readability overall. It also wins back a level of
indentation.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40294