samples: unicast_audio_client: Fix enabling the streams

This fixes the application that was not waiting for QoS configuration of
all streams in the group. The QoS operation is done on group of streams,
while the status of each one of the streams is received separately.
Meaning that the application has to control which streams were already
configured before enabling them.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>