i2s: Fix buffer size off by one error

The FIFO ring buffer cursors were not incrementing and creating an
illusion of a false LIFO queue because the mask and size were off.
Additionally, to ease debugging, this replaces the square wave of
the original test data with a more deterministic, and somewhat
shorter sine wave that was programmatically generated without
aliasing.

Change-Id: I118cc73bca6d467b967f702465c5f3a758ff4d07