Bluetooth: Make hci_le_conn_update return the store hint

The caller of hci_le_conn_update is directly interested in knowing what
the best value is for the store_hint parameter of the corresponding
mgmt event. Since hci_le_conn_update knows whether there were stored
parameters that were updated or not we can have it return an initial
store_hint value to the caller.

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