Fix dasync engine

The aes128_cbc_hmac_sha1 cipher in the dasync engine is broken. Probably
by commit e38c2e85

 which removed use of the "enc" variable...but not
completely.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8291)

(cherry picked from commit 695dd3a3)