pk: align library and tests code to the new internal functions

Note = programs are not aligned to this change because:
- the original mbedtls_pk_ec is not ufficially deprecated
- that function is used in tests when ECP_C is defined, so
  the legacy version of that function is available in that
  case

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index d94d829..0940bdb 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -2007,7 +2007,7 @@
         return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
     }
 
-    peer_key = mbedtls_pk_ec(*peer_pk);
+    peer_key = mbedtls_pk_ec_ro(*peer_pk);
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     size_t olen = 0;