Bluetooth: Fix DHKey Check sending order for slave role

According to the LE SC specification the initiating device sends its
DHKey check first and the non-initiating devices sends its DHKey check
as a response to this. It's also important that the non-initiating
device doesn't send the response if it's still waiting for user input.
In order to synchronize all this a new flag is added.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>