usb: device_next: tweak USB device notification support

Since no threads are waiting for the FIFO and we do not need any k_fifo
features, we can just use slist.

Some notifications (caused by interrupts), especially "VBUS ready", can
be triggered very quickly right after UDC initialization if the USB
controller is already connected. Depending on the global thread
configuration, messsage publishing work can be executed before the
context is marked as initialized. Use delayed work to reschedule
publishing when the context is not initialized.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>