openssl: Use ECDH_compute_key() for 'x-coordinate only' setting

openssl: Use ECDH_compute_key() for 'x-coordinate only' setting

ECDH_compute_key() was not used because it only gives x-coordinate of
the result. However, the default setting, as per the errata mentioned,
is to use x-coordinate only.
Use ECDH_compute_key() for this setting as it additionally allows HW
offload of the computation using dynamic engine feature in OpenSSL.
EC_POINT_mul() doesn't allow HW offload.

Signed-off-by: Mahantesh Salimath <mahantesh@nvidia.com>