Bluetooth: BAP: Shell: Set default_stream on configured

Instead of setting the default_stream when we initiate
the configure operatio, we set it when it has succeded.

In case that the codec configure fails, the stream should
not really be considered configured as the default.

This also ensures that the default_stream is set when
using the CAP shell commands.

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