OCSP/SNI: set correct callback.

Caught by Jeremy; was wrong in (my) original commit, the dual-TLS work
had just renamed the variables and theoretically made it more visible.
I still missed it.

The server_sni context initialisation was setting the OCSP status
callback context parameter back on the original server_ctx instead of
the new server_sni context.

I guess OCSP and SNI aren't being used together in Exim much yet.