crypto: ecc - remove checks in crypto_ecdh_shared_secret() and ecc_make_pub_key()

With the current state of the code, ecc_get_curve() cannot return
NULL in crypto_ecdh_shared_secret() and ecc_make_pub_key(). This is
conditioned by the fact that they are only called from ecdh_compute_value(),
which implements the kpp_alg::{generate_public_key,compute_shared_secret}()
methods. Also ecdh implements the kpp_alg::init() method, which is called
before the other methods and sets ecdh_ctx::curve_id to a valid value.

Signed-off-by: Roman Smirnov <r.smirnov@omp.ru>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>