[rescue] Update default rescue enter delay to 15s

The default delay for entering rescue mode was increased from 5s to 15s
to provide more time for the operation on CI environment.

Change-Id: If7b8de5656f2a90855962199612bff6e2e85fbf7
Signed-off-by: Yi-Hsuan Deng <yhdeng@google.com>

Author: sasdf

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

@@ -70,7 +70,7 @@ pub struct RescueParams {
     pub uart: UartParams,
     #[arg(long)]
     pub usb_serial: Option<String>,
-    #[arg(long, value_parser = humantime::parse_duration, default_value = "5s")]
+    #[arg(long, value_parser = humantime::parse_duration, default_value = "15s")]
     pub enter_delay: Duration,
 }