dm: sound: Use the correct number of channels for sound

At present the 'beep' sound generates a waveform for only one channel even
if two are being used. This means that the beep is twice the frequency it
should be. Correct this by making it a parameter.

The fix in a previous commit was correct for sandbox but not for other
boards.

Fixes: 03f11e87 ("sound: Correct data output in sound_create_square_wave()")
Signed-off-by: Simon Glass <sjg@chromium.org>