Further parallelize USB init by launching a thread per usb port.

Further parallelize USB init by launching a thread per usb port.

Run a thread per usb port in addition to the existing thread per usb
controller.  This can reduce total boot time by allowing multiple USB
devices on the same controller to initialize in parallel.  It also
makes startup time for critical devices (eg, the keyboard) less
dependent on which port they are plugged into.