rfc2217: do not set logger = None inside open()

[Qlexio]

The logger field is already initialized inside __init__ so no need to do it again.

It is possible for the user to initialize logger after __init__ and before open(), if that happens then we should log all negotiation messages. Right now they are always lost, it is impossible to log negotiation messages.

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This issue has been cloned from: pyserial#764