[powermax] Improve debugging in message classes (#9314)

[powermax] Improve debugging in message classes (#9314)

* Improve debugging in message classes
Each PowermaxBaseMessage subclass had its own toString() which added
useful debug info but duplicated a lot of the logic in the message
handler. I've moved the debug info inline and removed the duplication,
and also added decoding for more values to the debug output.
* Fix for 0xFF byte values, which convert to int -1
* Change to inline debug messages

Signed-off-by: Ron Isaacson <isaacson.ron@gmail.com>