crypto: mxs-dcp - Ensure payload is zero when using key slot

In the Linux kernel, the following vulnerability has been resolved: crypto: mxs-dcp - Ensure payload is zero when using key slot We could leak stack memory through the payload field when running AES with a key from one of the hardware's key slots. Fix this by ensuring the payload field is set to 0 in such cases. This does not affect the common use case when the key is supplied from main memory via the descriptor payload.