The USB subsystem has a few counters that need to be reset since they are
stored in static variables rather than driver-model data. An example is
usb_max_devs. Ultimately we should move this data into the USB uclass.

For now, make sure that USB is reset after each test, so that the counters
go back to zero.

Note: this is not a perfect solution: It a USB test fails it will exit
immediately and leave USB un-reset. The impact here is that it may cause
subsequence test failures in the same run.

Signed-off-by: Simon Glass <sjg@chromium.org>