crypto: sahara - fix AES descriptor create

The AES implementation still assumes, that the hw_desc[0] has a valid
key as long as no new key needs to be set; consequentialy it always
sets the AES key header for the first descriptor and puts data into
the second one (hw_desc[1]).

Change this to only update the key in the hardware, when a new key is
to be set and use the first descriptor for data otherwise.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>