Bluetooth: Audio: Fix some bad guards in audio shell

Some functions are only used for the unicast server
but was guarded by CONFIG_BT_AUDIO_UNICAST instead of
CONFIG_BT_AUDIO_UNICAST_SERVER.

Some parts of the unicast-server-only code also had
client specific code that would never be called.

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