ASoC: Fix resource leak in smdk_spdif.c

Properly free allocated resources in smdk_init() error path.
Add missing platform_device_unregister() in smdk_exit().

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Jassi Brar <jassi.brar@samsung.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>