usb: dwc3: remove custom unique id handling

The lockless implementation of the unique id is quite impressive (:P)
but dirver's core can handle it, we can remove it and make our code a
little smaller.

Cc: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>