speed.c: simplify aggregation of dsa --multi results

CLA: trivial

Signed-off-by: Cristian Stoica <cristian.stoica@nxp.com>

Reviewed-by: Geoff Thorpe <geoff@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1377)