Fix "int vs enum" warnings from armcc v5

enumerated type mixed with another type
diff --git a/library/cipher_wrap.c b/library/cipher_wrap.c
index 7b597d6..736c292 100644
--- a/library/cipher_wrap.c
+++ b/library/cipher_wrap.c
@@ -1442,7 +1442,7 @@
     { POLARSSL_CIPHER_NULL,                 &null_cipher_info },
 #endif /* POLARSSL_CIPHER_NULL_CIPHER */
 
-    { 0, NULL }
+    { POLARSSL_CIPHER_NONE, NULL }
 };
 
 #define NUM_CIPHERS sizeof cipher_definitions / sizeof cipher_definitions[0]