ASoC: Make SMDK WM8580 driver unloadable

Supply a module exit function so that the driver can be unloaded.

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