crypto: caam - don't dma_map key for hash algorithms

Shared descriptors for hash algorithms are small enough
for (split) keys to be inlined in all cases.
Since driver already does this, all what's left is to remove
unused ctx->key_dma.

Fixes: 045e3678 ("crypto: caam - ahash hmac support")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>