sound: Fix buffer overflow in square wave generation

Data is written for each channel but is only tracked as having one
channel written. This resulted in a buffer overflow and corruption of
the allocator's metadata which caused further problems when the buffer
was later freed. This could be observed with sandbox unit tests.

Resolve the overflow by tracking the writes for each channel.

Fixes: f987177d ("dm: sound: Use the correct number of channels for sound")
Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>