IdleLEDs: Explicitly cast idle_time_limit to uint32_t

When calculating `end_time`, explicitly cast `idle_time_limit` to 32 bits,
otherwise the `idle_time_limit * 1000` operation will be done on 16 bits, which
would overflow at about 65 seconds. With the cast, the operation will use all 32
bits, and we avoid the overflow.

Many thanks to @nevd for the report and the help in debugging & testing the fix.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/plugin/IdleLEDs.cpp

@@ -43,7 +43,7 @@ EventHandlerResult IdleLEDs::onKeyswitchEvent(Key &mapped_key, byte row, byte co
     ::LEDControl.refreshAll();
   }
 
-  end_time_ = Kaleidoscope.millisAtCycleStart() + idle_time_limit * 1000;
+  end_time_ = Kaleidoscope.millisAtCycleStart() + (uint32_t)idle_time_limit * 1000;
 
   return EventHandlerResult::OK;
 }