OMAPDSS: HDMI: Split audio_enable into audio_enable/disable

To improve readability, split the audio_enable HDMI IP operation
into two separate functions for enabling and disabling audio.
The audio_enable function is also modified to return an error value.

While there, update these operations for the OMAP4 IP accordingly.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
diff --git a/drivers/video/omap2/dss/dss_features.c b/drivers/video/omap2/dss/dss_features.c
index 1d10a01..3a28c5c 100644
--- a/drivers/video/omap2/dss/dss_features.c
+++ b/drivers/video/omap2/dss/dss_features.c
@@ -575,6 +575,7 @@
 #if defined(CONFIG_SND_OMAP_SOC_OMAP4_HDMI) || \
 	defined(CONFIG_SND_OMAP_SOC_OMAP4_HDMI_MODULE)
 	.audio_enable		=       ti_hdmi_4xxx_wp_audio_enable,
+	.audio_disable		=       ti_hdmi_4xxx_wp_audio_disable,
 #endif
 
 };