ASoC: mid-x86: Fix dependency on intel_sst driver

Enabling medfield asoc driver causes compliation error when intel_sst
is not selected
ERROR: "register_sst_card" [sound/soc/mid-x86/snd-soc-sst-platform.ko]
undefined!

This patch puts proper dependency to elimate build error

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Harsha Priya <priya.harsha@intel.com>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>