ASoC: hdmi-codec: stream is already locked in hw_params

startup() should have run before hw_params() is called, so the
current_substream pointer should already be properly set. There
is no reason to call hdmi_codec_new_stream() again in the
hw_params() callback

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>