Don't directly access iv_size

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/src/test_helpers/ssl_helpers.c b/tests/src/test_helpers/ssl_helpers.c
index 6027671..8e67352 100644
--- a/tests/src/test_helpers/ssl_helpers.c
+++ b/tests/src/test_helpers/ssl_helpers.c
@@ -1142,7 +1142,7 @@
     /* Pick cipher */
     cipher_info = mbedtls_cipher_info_from_type(cipher_type);
     CHK(cipher_info != NULL);
-    CHK(cipher_info->iv_size <= 16);
+    CHK(mbedtls_cipher_info_get_iv_size(cipher_info) <= 16);
     CHK(mbedtls_cipher_info_get_key_bitlen(cipher_info) % 8 == 0);
 
     /* Pick keys */
@@ -1273,7 +1273,7 @@
 
     /* Pick IV's (regardless of whether they
      * are being used by the transform). */
-    ivlen = cipher_info->iv_size;
+    ivlen = mbedtls_cipher_info_get_iv_size(cipher_info);
     memset(iv_enc, 0x3, sizeof(iv_enc));
     memset(iv_dec, 0x4, sizeof(iv_dec));