USB: eliminate panics in device_enumerator hotplug path

device_enumerator.rs:
- Line 31: panic!() on channel disconnect → graceful log+return
  (channel disconnect means xhcid is shutting down — graceful exit)
- Line 70: panic!() on port not in disabled state → warn+continue
  (transient power state during USB 2.0 port reset — skip and retry)

The device enumerator is the hotplug event consumer — it receives
PortStatusChange events from the IRQ reactor and calls attach_device()
for enumeration + spawn_drivers() for class driver spawning.  These
panic sites were the last remaining crash vectors in the hotplug path.

drivers/usb/xhcid/src/xhci/device_enumerator.rs

@@ -28,7 +28,8 @@ impl<const N: usize> DeviceEnumerator<N> {
             let request = match self.request_queue.recv() {
                 Ok(req) => req,
                 Err(err) => {
-                    panic!("Failed to received an enumeration request! error: {}", err)
+                    log::error!("Device enumerator channel disconnected: {} — exiting", err);
+                    return;
                 }
             };
 
@@ -68,10 +69,11 @@ impl<const N: usize> DeviceEnumerator<N> {
                         && !flags.contains(PortFlags::PR);
 
                     if !disabled_state {
-                        panic!(
+                        warn!(
-                            "Port {} isn't in the disabled state! Current flags: {:?}",
+                            "Port {} isn't in the disabled state! Current flags: {:?} — skipping enumeration",
                             port_id, flags
                         );
+                        continue;
                     } else {
                         debug!("Port {} has entered the disabled state.", port_id);
                     }