wallet_rpc_server: allow creating more than 64 addresses at once

it's too low a limit (at least one person mentioned having to
call create_address in a loop due to it)

src/wallet/wallet_rpc_server.cpp

@@ -576,9 +576,9 @@ namespace tools
     if (!m_wallet) return not_open(er);
     try
     {
-      if (req.count < 1 || req.count > 64) {
+      if (req.count < 1 || req.count > 65536) {
         er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
-        er.message = "Count must be between 1 and 64.";
+        er.message = "Count must be between 1 and 65536.";
         return false;
       }