ASoC: core: set_tdm_slot() will return -ENOTSUPP if no operation provided

Make it easier for generic code to work with set_tdm_slot() by distinguishing
between the operation not being supported and an error as is done.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>