pk: fix build issues

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 38a3fc4..aa3e306 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -2636,7 +2636,7 @@
         case MBEDTLS_PK_ECKEY:
         case MBEDTLS_PK_ECKEY_DH:
         case MBEDTLS_PK_ECDSA:
-            key = mbedtls_pk_ec_ro(*pk);
+            key = mbedtls_pk_ec_rw(*pk);
             if (key == NULL) {
                 return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
             }