Bluetooth: hci_conn: Fix hci_le_set_cig_params

When running with concurrent task only one CIS was being assigned so
this attempts to rework the way the PDU is constructed so it is handled
later at the callback instead of in place.

Fixes: 26afbd82

 ("Bluetooth: Add initial implementation of CIS connections")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>