Clamp too-large hwpmc callchaindepth to the maximum

If the depth requested by the user is too large, it's better to provide
the maximum than the smaller default.

Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274766 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f