ASoC: Intel: Skylake: Fix default dma_buffer_size

If the dma_buffer_size is not defined in topology, fix it to 2ms default
value to make backward compatible.

Fixes: f6e6ab1d ("ASoC: Intel: Skylake: Fix dma buffer size calculation")
Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>