When several stm32usb serial consoles are present, comare each USB
device serial number with the micro servo to find the correct one
rather than using the usb.util.get_string() method which appears to be
doing something completely different and never returns a matching
serial number.  The Servo Micro uses such a console, so when
connecting more than one to the same controller the issue occurs.

This issue cannot be seen when only one servo board has a stm32usm
serial device.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>