net: cdc_ncm: error path lock fix

Fixes the sparse warning

 drivers/net/usb/cdc_ncm.c:836:9: warning: context imbalance in 'cdc_ncm_txpath_bh' - different lock contexts for basic block
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_ncm.c

@@ -819,6 +819,8 @@ static void cdc_ncm_txpath_bh(unsigned long param)
 		netif_tx_lock_bh(ctx->netdev);
 		usbnet_start_xmit(NULL, ctx->netdev);
 		netif_tx_unlock_bh(ctx->netdev);
+	} else {
+		spin_unlock_bh(&ctx->mtx);
 	}
 }