ASoC: hdmi: Mark the maximum significant bits to HDMI codec

HDMI audio can not have more than 24 bits even if on i2s bus there
would be 32 bit samples. Mark this by adding .sig_bits = 24 to
playback stream definition.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>