[ALSA] remove redundent assignment to the ac97 device structure

AC97 Codec
Don't use dev.platform_data to store a reference to the containing
ac97_t structure.  Such assignment is redundent since we can deduce the
ac97_t structure location from the contained device structure.  This
sets platform_data free for other purposes.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/ac97_codec.h b/include/sound/ac97_codec.h
index 2857cf0..d11f348 100644
--- a/include/sound/ac97_codec.h
+++ b/include/sound/ac97_codec.h
@@ -527,6 +527,8 @@
 	struct device dev;
 };
 
+#define to_ac97_t(d) container_of(d, struct _snd_ac97, dev)
+
 /* conditions */
 static inline int ac97_is_audio(ac97_t * ac97)
 {