sst: make register_sst_card more self-contained

register_sst_card is used in ASoC code with field `scard_ops` being NULL.
Without this patch, there will be NULL dereference.

Signed-off-by: Lu Guanqun <guanqun.lu@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>