crypto: sun4i-ss - Always use sun4i_tfm_ctx for storing pointer to dev ss

The dev *ss is stored both in sun4i_tfm_ctx and sun4i_req_ctx.
Since this pointer will never be changed during tfm life, it is better
to remove it from sun4i_req_ctx.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>