ASoC: Constify the 'driver' field of snd_soc_platform

The ASoC core does no not modify the driver of a platform. Making it const
allows ASoC platform drivers to declare the snd_soc_platform_driver struct as
const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index 0ce075c..4d24b5e 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -3906,7 +3906,7 @@
  * @platform: platform to register
  */
 int snd_soc_register_platform(struct device *dev,
-		struct snd_soc_platform_driver *platform_drv)
+		const struct snd_soc_platform_driver *platform_drv)
 {
 	struct snd_soc_platform *platform;