drm/vc4/vc4_hdmi: Remove set but unused variable 'ret'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vc4/vc4_hdmi.c: In function ‘vc4_hdmi_set_audio_infoframe’:
 drivers/gpu/drm/vc4/vc4_hdmi.c:334:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Eric Anholt <eric@anholt.net>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Rob Clark <robdclark@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-39-lee.jones@linaro.org