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.
Bluetooth: Fix letting the remote do too many passkey rounds

There should be at most 20 passkey entry rounds, and if the remote tries
to do more we should just ignore the PDUs.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>