drivers: usb_dc_nrfx: Use dedicated work queue for handling ISR events

This patch introduces a dedicated work queue for handling the events
from ISR (i.e. for notifying the USB device stack, for executing the
enpoints callbacks, etc.). The system work queue cannot be used for
this purpose as it might be used in applications for scheduling USB
transfers and this could lead to a deadlock when the USB device stack
would not be notified about certain event because of a system work
queue item waiting for a USB transfer to be finished.

The FIFO named so far `work_queue` is renamed to `usbd_evt_fifo`
to better indicate its purpose and to avoid confusion.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>