drm/omap: Remove set but not used variable 'err' in hdmi4_audio_config

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/omapdrm/dss/hdmi4_core.c: In function hdmi4_audio_config:
drivers/gpu/drm/omapdrm/dss/hdmi4_core.c:689:6: warning: variable err set but not used [-Wunused-but-set-variable]

It is not used since commit f5bab222

 ("OMAPDSS:
HDMI: Add OMAP5 HDMI support")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1570518949-47574-5-git-send-email-zhengbin13@huawei.com