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

sound/soc/sprd/Kconfig

@@ -8,7 +8,7 @@ config SND_SOC_SPRD
 	  the Spreadtrum SoCs' Audio interfaces.
 
 config SND_SOC_SPRD_MCDT
-	bool "Spreadtrum multi-channel data transfer support"
+	tristate "Spreadtrum multi-channel data transfer support"
 	depends on SND_SOC_SPRD
 	help
 	  Say y here to enable multi-channel data transfer support. It

sound/soc/sprd/sprd-mcdt.h

@@ -48,7 +48,7 @@ struct sprd_mcdt_chan {
 	struct list_head list;
 };
 
-#ifdef CONFIG_SND_SOC_SPRD_MCDT
+#if IS_ENABLED(CONFIG_SND_SOC_SPRD_MCDT)
 struct sprd_mcdt_chan *sprd_mcdt_request_chan(u8 channel,
 					      enum sprd_mcdt_channel_type type);
 void sprd_mcdt_free_chan(struct sprd_mcdt_chan *chan);