`spdm_key_exchange_response_t.req_slot_id_param` does not match specification

[steven-bellock]

It should just be slot_id_param.

[jyao1]

I purposely rename it from slot_id_param to req_slot_id_param.

It is very easy to make mistake in the code, if we do not put req_ .

I do not want to change name in the code. We can add comment to clarify.