ASoC: hdmi-codec: use sizeof_field rather than declaring hcp

Use FIELD_SIZEOF rather than declaring and initializing hcp. Remove
unused variables. Cleans up clang warning:

warning: Value stored to 'hcp' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>