crypto: Don't assert on valid IV length for Chacha20-Poly1305.

The assertion checking for valid IV lengths added in 1833d6042c9a
was not properly updated to permit an IV length of 8 in commit
42dcd39528c6.

Reported by: syzbot+f0c0559b8be1d6eb28c7@syzkaller.appspotmail.com
Reviewed by: markj
Fixes: 42dcd39528c6 crypto: Support Chacha20-Poly1305 with a nonce size of 8 bytes.
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32860