DPP: Recognize own PKEX Exchange Request if it ends up being received

DPP: Recognize own PKEX Exchange Request if it ends up being received

It is possible for a Controller to receive a copy of its own PKEX
Exchange Request in the case where the Controller is initiating a PKEX
exchange through a Relay. The Configurator role in the device would have
a matching PKEX code in that case and the device might reply as a PKEX
responder which would result in going through the exchange with the
Controller device itself. That is clearly not desired, so recognize this
special case by checking whether the Encrypted Key attribute value
matches a pending locally generated one when processing a received PKEX
Exchange Request.

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>