devctl_process_running(): Check for devsoftc.inuse == 1 instead of
devsoftc.async_proc != NULL because the latter might not be true
sometimes.
This way /etc/rc.suspend gets executed.

Reviwed by:     njl
Submitted by:   Mitsuru IWASAKI <iwasaki at jp.FreeBSD.org>
Tested also by: Andreas Wetzel <mickey242 at gmx.net>