ASoC: sprd: Allow the MCDT driver to build into modules

Change the config to 'tristate' for MCDT driver to allow it to build into
modules, as well as changing to use IS_ENABLED() to validate if need supply
dummy functions when building the MCDT driver as a module.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/9306f2b99641136653ae4fe6cf9e859b7f698f77.1583387748.git.baolin.wang7@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>