However I've added 3 new ioctl's (and I'm unsure how to define their
   exact numbers). [...]

That's ... special, and silly.  If you're going to do this at all,
it should be done in a device-independent fashion.  I'd suggest 
adding a TCSADEFAULT (or some such thing) type for the `action'
field to tcsetattr(), and otherwise reusing the same user API.