ASoC: wm9705: Fix build due to removal of 'runtime' definition

sound/soc/codecs/wm9705.c: In function 'ac97_prepare':
sound/soc/codecs/wm9705.c:251: error: 'runtime' undeclared (first use in this function)

This was caused by commit e6968a (ASoC: codecs: Remove rtd->codec usage from CODEC drivers),
which removed the 'struct snd_pcm_runtime *runtime = substream->runtime' definition.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/codecs/wm9705.c

@@ -248,7 +248,7 @@ static int ac97_prepare(struct snd_pcm_substream *substream,
 	else
 		reg = AC97_PCM_LR_ADC_RATE;
 
-	return ac97_write(codec, reg, runtime->rate);
+	return ac97_write(codec, reg, substream->runtime->rate);
 }
 
 #define WM9705_AC97_RATES (SNDRV_PCM_RATE_8000 | SNDRV_PCM_RATE_11025 | \