Fix hwpmc sampling for ppc970 (G5-class) processors.

With this, hwpmc sampling now works on these processors.

MFC after:	3 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275190 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Revert part of r275059. Comparing unsigned 8 bit value
against -'0' is always false so the conditional block is
optimized away.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275191 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f