RADIUS client: Do not flush pending messages if server did not change

RADIUS client: Do not flush pending messages if server did not change

The re-open socket to the current RADIUS server code path did not work
in the expected way here. The pending authentication messages do not
need to be flushed in that case and neither should the retransmission
parameters be cleared. Fix this by performing these operations only if
the server did actually change as a part of a failover operation.

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