Bugfix for racy crash on opening bad serial port

[unjambonakap]

To reproduce: on linux, try to connect to an existing /dev/ttySX that is not a valid serial port (preexisting ttys?)

Crash happens like this:

• Open an existing port that triggers on open a QSerialPort::ResourceError (stock /dev/ttySX on my end)
• Signal QSerialPort::errorOccurred
• Slot SerialLink::linkError
• LinkInterface::_connectionRemoved
• SerialLink::disconnect => set _port = null
• On retry, at [https://github.com/unjambonakap/qgroundcontrol/blob/aacb664bffc2baa47c6f6a8ec9a224149fd06977/src/Comms/SerialLink.cc#L182], crashes because _port == null

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Simply moves the connection of signals (readyRead and errorOccurred) until the serial port's configuration is finished.

Note: I'm guessing that should data arrive in between port opening and connection of the readyRead() signal, it does not matter since it's the event loop stuff that triggers the readyRead()