Bluetooth: Iso: Change set CIG parameters fail to unref

Change the fail handling of setting the CIG parameters
(or when allocating the CIS) to unref the ISO conns
instead of calling cleanup on them directly.

This fixes an issue that if the CIG parameters failed,
then the iso conns would not be unref'ed and we could
not try again.

Unref'ing an iso conn will call cleanup once it hits 0
refs.

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