fix cppcheck warning:

sound/soc/codecs/hdmi-codec.c:25:16: style: struct member
'hdmi_codec_channel_map_table::spk_mask' is never
used. [unusedStructMember]
 unsigned long spk_mask;  /* speaker position bit mask */
               ^
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210312182246.5153-12-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>