crypto: rockchip - better handle cipher key

The key should not be set in hardware too much in advance, this will
fail it 2 TFM with different keys generate alternative requests.
The key should be stored and used just before doing cipher operations.

Fixes: ce0183cb ("crypto: rockchip - switch to skcipher API")
Reviewed-by: John Keeping <john@metanate.com>
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>