IOS/USB: Fix initial device scan

Even though libusb is supposed to be thread-safe, in practice
it's not (at least on Windows); getting a list of devices from two
different threads can result in libusb crashes. This is easily
fixed by waiting for the scan thread to complete scanning instead
of running the scan on the CPU thread.

This also fixes an issue that I had overlooked in the initial
implementation: IOS interfaces such as OH0 are sometimes opened
every frame, in which case we were doing a full device scan every
single frame on the CPU thread!

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