ASoC: soc-cache: Return -ENOSYS instead of -EINVAL

These functions fail with -EINVAL if the corresponding callbacks
are not implemented.  Change them to return -ENOSYS as it is more
appropriate for unimplemented callbacks.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>