drivers: usb_dc_nrfx: Do not uninit on detach

It is possible for nrfx usbd driver to send NRFX_USBBD_EP_ABORTED during
endpoint disable. The event is passed to event handler registered with
nrfx_usbd_init(). The nrfx_usbd_uninit() removes the registered event
handler, replacing it with NULL. If any event is sent after uninit, the
NULL pointer is executed and device crashes.

Do not uninit nrfx usbd driver on detach so it is possible for the
usb_disable() to disable all the endpoints.

Fixes: 460ca865 ("drivers: usb_dc_nrfx: Always allow endpoint disable")

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>