Bluetooth: ISO: Fix issue with bt_iso_cig_reconfigure

bt_iso_cig_reconfigure would almost always fail due to invalid
checks that did not take the functioner properly into account.

The CIS provided to bt_iso_cig_reconfigure can now be allocated
beforehand, if the CIG for the CIS is the same as the one being
reconfigured.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>