chachapoly: adjust parameter order

This module used (len, pointer) while (pointer, len) is more common in the
rest of the library, in particular it's what's used in the GCM API that
very comparable to it, so switch to (pointer, len) for consistency.

Note that the crypt_and_tag() and auth_decrypt() functions were already using
the same convention as GCM, so this also increases intra-module consistency.

library/cipher.c

@@ -320,7 +320,7 @@ int mbedtls_cipher_update_ad( mbedtls_cipher_context_t *ctx,
             return( result );
 
         return mbedtls_chachapoly_update_aad( (mbedtls_chachapoly_context*) ctx->cipher_ctx,
-                                                          ad_len, ad );
+                                                          ad, ad_len );
     }
 #endif