Bluetooth: Audio: Ensure that stream is in streaming state for ISO recv

Adds check to verify that the audio stream is actually
in the streaming state before handling the
audio data. As per the BAP spec, a stream that
is not in the streaming state shall not send
any data, and this check is to handle any
remote devices that are not working correctly.

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