ALSA: pcm_dmaengine: Add missing module description

Now that make W=1 starts complaining the lack of MODULE_DESCRIPTION(),
let's add the missing information.
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Closes: https://lore.kernel.org/r/ZjpQm-hxLQtpgkUx@smile.fi.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/20240508091128.25274-4-tiwai@suse.de

sound/core/pcm_dmaengine.c

@@ -470,4 +470,5 @@ int snd_dmaengine_pcm_refine_runtime_hwparams(
 }
 EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_refine_runtime_hwparams);
 
+MODULE_DESCRIPTION("PCM dmaengine helper APIs");
 MODULE_LICENSE("GPL");