drivers: udc_kinetis: add missing return statement

Add the missing return statement to the branch where no slabs are
available.

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

drivers/usb/udc/udc_kinetis.c

@@ -448,6 +448,8 @@ static void usbfsotg_event_submit(const struct device *dev,
 	ret = k_mem_slab_alloc(&usbfsotg_ee_slab, (void **)&ev, K_NO_WAIT);
 	if (ret) {
 		udc_submit_event(dev, UDC_EVT_ERROR, ret);
+		LOG_ERR("Failed to allocate slab");
+		return;
 	}
 
 	ev->dev = dev;