When we dropped the dedicated command argument frame type, this left
a hole in our frame type numbering. Let's start plugging that hole.
The command data frame is now type value 2 instead of 3.
There was limited test fallout because a) we do a good job of using
the constants to refer to frame types b) not many tests are sending
command data frames.
Bumping the media type will be performed in a later commit, once all
type value adjustment has been performed.