mark "inst" as unused, and relax checks a bit more

mark "inst" as unused, and relax checks a bit more

for now, we will only check if the EAP message length is correct.
This prevents "middle box" effect, where the protocol cannot be
upgraded, because middle boxes aren't upgraded.

There is a near-zero chance that EAP will be updated to allow
supplicants to send something other than 1 (Response).  But for
general protocol correctness and paranoia, let's allow for that