crypto/internal/fips140/aes: handle fallback correctly

crypto/internal/fips140/aes: handle fallback correctly

Don't fallthrough to the hardware version if we used the generic version.

This might fix the s390x build on the dashboard.

(Originally broken on CL 624738.)

Change-Id: Idad1f1973a34fc64550ecf0d012651f62bcd6272
Reviewed-on: https://go-review.googlesource.com/c/go/+/631315
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>