Bluetooth: Fix Add Device to wait for HCI before sending cmd_complete

This patch updates the Add Device mgmt command handler to use a
hci_request to wait for HCI command completion before notifying user
space of the mgmt command completion. To do this we need to add an extra
hci_request parameter to the hci_conn_params_set function. Since this
function has no other users besides mgmt.c it's moved there as a static
function.

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