ASoC: twl6040: make pointer dmic_codec_dev static

The pointer dmic_codec_dev is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'dmic_codec_dev' was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/omap/omap-abe-twl6040.c

@@ -40,7 +40,7 @@ struct abe_twl6040 {
 	int	mclk_freq;	/* MCLK frequency speed for twl6040 */
 };
 
-struct platform_device *dmic_codec_dev;
+static struct platform_device *dmic_codec_dev;
 
 static int omap_abe_hw_params(struct snd_pcm_substream *substream,
 	struct snd_pcm_hw_params *params)