Bluetooth: Audio: Shell: Refactor set_preset

The function is now called get_named_preset and only
returns a named preset. The QoS parameter handling
has been moved to a new function, cmd_stream_qos,
which can be used to set specific QoS for a stream.

The get_named_preset does also no longer set the default
preset, and cmd_preset must be used for that now.

This means that a function like cmd_config no longer
has any side effects, thus making it easier to set up
multiple streams with different presets.

Signed-off-by: Emil Gydesen <[email protected]>

Author: Thalley