Bluetooth: GATT: Fix not queuing writes to CCC

In order to properly queue request there need to be a bt_att_req
storage but none of the calls to gatt_write_ccc were using the params
causing gatt_send to use bt_att_send and not bt_att_req_send.

To fix this now all the callers of gatt_write_ccc do set the params
properly but this means that bt_gatt_unsubscribe has to wait for it
to be completed before the application can reuse the
bt_gatt_subscribe_params.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>