ASoC: sst_platform: using builtin function

Use the builtin snd_soc_set_runtime_hwparams() instead of assigning it by
myself.

Signed-off-by: Lu Guanqun <guanqun.lu@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>