[usb] Implement sync control transfer via async (#456)
[usb] Implement sync control transfer via async (#456)

Change our libusb_control_transfer implementation - which performs
synchronous transfer - to redirect to our asynchronous transfer
implementation.

This helps streamlining various USB transfer flows. When migrating to
the new libusb-to-JS adaptor, we'll only need to take care of the
asynchronous code path. It's preparatory work for the WebUSB
support effort tracked by #429.
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