opencrypto: Make cryptosoft attach silently

cryptosoft is always present and doesn't print any useful information
when it attaches.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29098

(cherry picked from commit 4fc60fa9294f82c7f4e1a0e71f9a17794124217f)