OWE: Reuse own DH private key in AP if STA tries OWE association again

OWE: Reuse own DH private key in AP if STA tries OWE association again

This is a workaround for mac80211 behavior of retransmitting the
Association Request frames multiple times if the link layer retries
(i.e., seq# remains same) fail. The mac80211 initiated retransmission
will use a different seq# and as such, will go through duplicate
detection. If we were to change our DH key for that attempt, there would
be two different DH shared secrets and the STA would likely select the
wrong one.

Signed-off-by: Jouni Malinen <j@w1.fi>