ALSA: core: allow SND_DMAENGINE_PCM use from modules

When users of SND_DMAENGINE_PCM are built as module, the config symbol
SND_DMAENGINE_PCM must be tristate, otherwise the linker will fail.
Signed-off-by: NDaniel Mack <zonque@gmail.com>
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

sound/core/Kconfig

@@ -7,7 +7,7 @@ config SND_PCM
 	select SND_TIMER
 
 config SND_DMAENGINE_PCM
-	bool
+	tristate
 
 config SND_HWDEP
 	tristate