fix(llcc68): cache detected chip type

Store the detected chip type after the first successful version-register read so repeated radio operations do not keep querying the chip identity.

This preserves Unknown as uncached so transient read failures or unsupported responses can still be retried on a later call.

include/llcc68.hpp

@@ -303,6 +303,7 @@ struct LLCC68 {
 
 	/** properties */
 	const struct device *dev;
+	std::optional<ChipType> cached_chip_type{};
 };
 } // namespace app::driver::llcc68