drivers: nordic: usb: USBD event handling rework

Some of the events from USBD peripheral (i.e. cable disconnect)
were handled in IRQ context and some of them (i.e. ep r/w events) in
system workqueue (inherited from initial driver implementation).
This may lead to race condition in some specific situations.
Currently, all of the events are enqueued in ISR and processed in
workqueue. Driver is reinitialized on queue overflow and queue size
is configurable in KConfig.

Fixes #12016

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>