wallet2: use BP+ for cold signing

reported by ukoehb

src/wallet/wallet2.cpp

@@ -10595,7 +10595,7 @@ void wallet2::cold_sign_tx(const std::vector<pending_tx>& ptx_vector, signed_tx_
   hw::wallet_shim wallet_shim;
   setup_shim(&wallet_shim, this);
   aux_data.tx_recipients = dsts_info;
-  aux_data.bp_version = (use_fork_rules(HF_VERSION_CLSAG, -10) ? 3 : use_fork_rules(HF_VERSION_SMALLER_BP, -10) ? 2 : 1);
+  aux_data.bp_version = (use_fork_rules(HF_VERSION_BULLETPROOF_PLUS, -10) ? 4 : use_fork_rules(HF_VERSION_CLSAG, -10) ? 3 : use_fork_rules(HF_VERSION_SMALLER_BP, -10) ? 2 : 1);
   aux_data.hard_fork = get_current_hard_fork();
   dev_cold->tx_sign(&wallet_shim, txs, exported_txs, aux_data);
   tx_device_aux = aux_data.tx_device_aux;