Fix GATT derive visibility

[vE5li]

I noticed that when deriving gatt_client, the methods would always be private. I implemented the same logic as for gatt_server, meaning the methods will be public if the field itself is public.

Edit: forgot to mention that I also noticed a single method on gatt_service not having the expected visibility, so I changed that as well.

[Dirbaio]

Makes sense, thank you! :)