crypto/internal/fips140/aes: handle fallback correctly, take 2

crypto/internal/fips140/aes: handle fallback correctly, take 2

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

Missed one case of this in CL 631315.

(Originally broken on CL 624738.)

Change-Id: I5bf485ac8b0007769e4e935794defe2a3592376e
Reviewed-on: https://go-review.googlesource.com/c/go/+/631136
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>