Use fewer bits for block_size

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/nist_kw.c b/library/nist_kw.c
index d15c5c4..fbd7221 100644
--- a/library/nist_kw.c
+++ b/library/nist_kw.c
@@ -75,7 +75,7 @@
         return MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA;
     }
 
-    if (cipher_info->block_size != 16) {
+    if (mbedtls_cipher_info_get_block_size(cipher_info) != 16) {
         return MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA;
     }