fix edgetpu device type "None" not working

[Links2004]

fix edgetpu device type "None" (auto)

from docu:
https://docs.frigate.video/configuration/detectors#edge-tpu-detector

The EdgeTPU device can be specified using the "device" attribute 
according to the Documentation for the TensorFlow Lite Python API.
If not set, the delegate will use the first device it finds.

but the code currently forces the device type to usb, which will not detect pci devices.
this is a problem in hybrid use cases where sometimes a pci and sometimes a USB device is available.

based on pycoral:
https://github.com/google-coral/pycoral/blob/ae743b0b234d66d784826ccfbb4607a69dc1a2a8/pycoral/utils/edgetpu.py#L87C32-L87C32

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