Bluetooth: Audio: Shell: Fix issue with restarting start_sine

The lc3_start_time and lc3_sdu_cnt were not reset properly,
causing it to fail when trying to do start_sine after calling
stop_sine.

This commit also ensures that the sending of the sine wave is
stopped once the stream is either released or stopped.

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