crypto: caam - fix inconsistent assoc dma mapping direction

req->assoc is dma mapped BIDIRECTIONAL and unmapped TO_DEVICE.
Since it is read-only for the device, use TO_DEVICE both for mapping
and unmapping.

Cc: <stable@vger.kernel.org> # 3.9, 3.8
Signed-off-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>