[otlib,rescue] Increase USBDFU enter delay

I have observed some very large delays in CI under heavy loads
where it takes close to 10 seconds before the device is available
in the container.

Signed-off-by: Amaury Pouly <amaury.pouly@opentitan.org>

Author: pamaury

sw/host/opentitanlib/src/rescue/usbdfu.rs

@@ -29,7 +29,7 @@ impl UsbDfu {
             interface: Cell::default(),
             params,
             reset_delay: Duration::from_millis(50),
-            enter_delay: Duration::from_secs(5),
+            enter_delay: Duration::from_secs(20),
         }
     }