test/py: dfu: wait for USB device to go away at boot

It can take a while for a host machine to notice that a USB device has
disconnected, and process the change. At the end of the DFU test, we wait
up to 10 seconds for this to happen. This change makes the test wait the
same (up to) 10 seconds at the start of the test for any previously active
USB device-mode session to be cleaned up. Such as session might have been
used to download U-Boot into memory for example; this is certainly true
on my Tegra test systems. This changes should solve the DFU test
intermittency issues I've been seeing on some Tegra devices.

Signed-off-by: Stephen Warren <swarren@nvidia.com>