Bluetooth: Audio: Fix issue with creating unidirectional CIS

As the unicast client, we would always create bidirectional
CIS to ensure that the PHY parameter is correctly set.
We can, however, just set the (required) PHY value and
leave the SDU and RTN values as 0, to avoid
creating bidirectional CIS.

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