ASoC: sti-sas: remove unused struct members

ASoC: sti-sas: remove unused struct members

cppcheck warnings:

sound/soc/codecs/sti-sas.c:54:25: style: struct member
'sti_dac_audio::field' is never used. [unusedStructMember]
 struct regmap_field  **field;
                        ^

sound/soc/codecs/sti-sas.c:55:24: style: struct member
'sti_dac_audio::rst' is never used. [unusedStructMember]
 struct reset_control *rst;
                       ^

sound/soc/codecs/sti-sas.c:61:25: style: struct member
'sti_spdif_audio::field' is never used. [unusedStructMember]
 struct regmap_field  **field;
                        ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210312182246.5153-19-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>