ASoC: SOF: Intel: hda: set bus->idx as 0

Setting the bus->idx as 0, for we only have one HDA
bus atm. This need to be fixed when there are more
than one HDA bus.

Signed-off-by: Zhu Yingjiang <yingjiang.zhu@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>