Serial_::write tries to send with closed host

[daniel-starke]

Serial_::write in CDC.cpp checks if the CDC connection is open by checking if DTR or RTS line state is set. The OS usually sets DTR on open and resets it on close. RTS on the other hand is often left as it is. This means that the check against both leads into the assumption of an valid connection even though there is no peer to read the data, which on the other hand leads into 250 ms delays in USB_Send after the send buffer got saturated. I suggest to check only if DTR is set.