crypto: omap-aes - fix context handling for multiple cores

AES can have multiple HW accelerator cores in the system, in which case
each core has its own crypto engine in use. Currently, the used hardware
device is stored under the omap_aes_ctx struct, which is global for
the algorithm itself, causing conflicts when used with multiple cores.
Fix this by moving the used HW device under reqctx, which is stored
per-request basis.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>