WPS: Use WSC_NACK if no device password is known on M2 RX

WPS: Use WSC_NACK if no device password is known on M2 RX

This can happen on the AP if the AP PIN is not configured and
the client tries to go through the protocol instead of just using
Registrar mode to receive M1 from the AP. It is cleaner to send
out the WSC_NACK instead of just stopping the protocol.