I'm sorry, I've no experience with compiling a driver for Linux.

Q: When I plug in the USB device without even using it, should  
ftdi_sio and usbserial remain loaded after I unplug the USB device?


Quoting Alan Cox [off-list ref]:

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The existence of /dev/ttyUSB0 depends on the system:
* RPi with Raspbian: /dev/ttyUSB0 never goes away after step 4.
* RPi with Debian6, BBXM: /dev/ttyUSB0 goes away after step 4 and
comes back after step 6.

Thats dependant upon your user space and really nothing to do with this

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Are there any commands I can run to provide you with more information?

As I said...

quoted

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If you think that if check is the problem can you replace it with an
unconditional call to change_speed(tty, port) and the re-test. That will
confirm if it is the cause.

Alan