rust/hpet: Fix a clippy error

Carge clippy complained about:

error: casts from `u8` to `u32` can be expressed infallibly using `From`

So use `From` to convert `u8` to `u32`.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250414144943.1112885-10-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/timer/hpet/src/hpet.rs

@@ -353,7 +353,7 @@ impl HPETTimer {
         // still operate and generate appropriate status bits, but
         // will not cause an interrupt"
         self.get_state()
-            .update_int_status(self.index as u32, set && self.is_int_level_triggered());
+            .update_int_status(self.index.into(), set && self.is_int_level_triggered());
         self.set_irq(set);
     }