ASoC: max98090: remove useless assignment

cppcheck warning:

sound/soc/codecs/max98090.c:1835:16: style: Variable 'test_diff' is
assigned a value that is never used. [unreadVariable]
 int test_diff = INT_MAX;
               ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210312182246.5153-13-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>