dragonfly: SAE/EAP-pwd min PWE derivation iteration count to shared code

dragonfly: SAE/EAP-pwd min PWE derivation iteration count to shared code

Use a shared function to determine the k parameter, i.e., the minimum
number of iterations of the PWE derivation loop, for SAE and EAP-pwd.
This makes it easier to fine-tune the parameter based on the negotiated
group, if desired.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>