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: Daniel Mack <zonque@gmail.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
diff --git a/sound/core/Kconfig b/sound/core/Kconfig
index 94ce1c4..313f22e 100644
--- a/sound/core/Kconfig
+++ b/sound/core/Kconfig
@@ -7,7 +7,7 @@
 	select SND_TIMER
 
 config SND_DMAENGINE_PCM
-	bool
+	tristate
 
 config SND_HWDEP
 	tristate