pk: use better naming for the new key ID field

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 3025725..ac6c10d 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -2614,7 +2614,7 @@
                 return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
             }
 
-            ssl->handshake->ecdh_psa_privkey = pk->opaque_id;
+            ssl->handshake->ecdh_psa_privkey = pk->priv_id;
 
             /* Key should not be destroyed in the TLS library */
             ssl->handshake->ecdh_psa_privkey_is_external = 1;