ASoC: nau8821: clarify out-of-bounds check

cppcheck reports a false positive

sound/soc/codecs/nau8821.c:390:17: error: Array 'dmic_speed_sel[4]'
accessed at index 4, which is out of bounds. [arrayIndexOutOfBounds]

  dmic_speed_sel[i].param, dmic_speed_sel[i].val);
                ^
sound/soc/codecs/nau8821.c:378:2: note: After for loop, i has value 4
 for (i = 0 ; i < 4 ; i++)
 ^
sound/soc/codecs/nau8821.c:390:17: note: Array index out of bounds
  dmic_speed_sel[i].param, dmic_speed_sel[i].val);
                ^

While the code is not incorrect, we can deal with the out-of-bounds
check in a clearer way that makes static analysis happy.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20211025185933.144327-5-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/nau8821.c

@@ -381,7 +381,7 @@ static int dmic_clock_control(struct snd_soc_dapm_widget *w,
 			speed_selection = dmic_speed_sel[i].val;
 			break;
 		}
-	if (speed_selection < 0)
+	if (i == 4)
 		return -EINVAL;
 
 	dev_dbg(nau8821->dev,