ossl: Don't try to initialize the cipher for Chacha20+Poly1305.

Chacha20+Poly1305 doesn't use an ossl_cipher instance the way AES-GCM
does, so ossl_lookup_cipher() failed causing ossl_newsession() to
always fail for Chacha20+Poly1305 sessions.

Reported by: gallatin (ktls_test fails with ossl.ko loaded)
Fixes: 9a3444d91c70 ossl: Add a VAES-based AES-GCM implementation for amd64
Tested by: gallatin
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40580