Make ctl_code const

[unlimitedsola]

So that it could be used in const context and precomputed at compile time e.g.:

const ACR122_CTL_CODE: DWORD = ctl_code(3500);

It should be a straightforward change and I'm willing to make a PR if needed.

Marking an existing function const shouldn't introduce any breakage as far as I can tell. In my opinion, the chance of this getting "un-constified" is very unlikely. What do you think?