[powermax] Refactor state objects (#9684)

[powermax] Refactor state objects (#9684)

* 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

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