Bluetooth: Host: Fix deadlock calling API functions from SMP callbacks

Fix deadlock in Bluetooth Host. Deadlock could happen from the SMP
callbacks when calling Bluetooth API functions. This is because the
callbacks was given directly from the HCI TX thread. If the calling
API function resulted in trying to send a new HCI command it would post
this HCI command to the HCI TX thread and then wait for command complete
event. This would result in the HCI TX thread blocked waiting for the
itself to process the command.

Example:
Calling bt_conn_le_conn_param_update from pairing_complete callback.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>