drivers: udc: add opaque pointer to store upper layer private data

Add an opaque pointer to store upper layer private data and initialize
it with the USB device context during controller initialization. Use the
pointer in event processing to get the correct context.

Fixes commit 48f2a4bc
("usb: device_next: remove initialized state checks in event processing")

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
(cherry picked from commit 5ddaa3b1)