ASoC: omap-hdmi-audio: Set buffer bytes step constraint to 128

Set buffer bytes step constraint to 128. A matching constraint has
already been set to period size. This helps PCM setup to tolerate ALSA
clients that set the PCM hw params in unusual order.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>