ASoC: wm8904: fix automatic sysclk configuration

The simple-card tries to signal the codec to disable rate constraints,
see commit 2458adb8 ("SoC: simple-card-utils: set 0Hz to sysclk when
shutdown"). This wasn't handled by the codec, instead it would set the
FLL frequency to 0Hz which isn't working. Since we don't have any rate
constraints just ignore this request.

Fixes: 13409d27 ("ASoC: wm8904: configure sysclk/FLL automatically")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20191122232532.22258-1-michael@walle.ccSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/wm8904.c

@@ -1806,6 +1806,12 @@ static int wm8904_set_sysclk(struct snd_soc_dai *dai, int clk_id,
 
 	switch (clk_id) {
 	case WM8904_CLK_AUTO:
+		/* We don't have any rate constraints, so just ignore the
+		 * request to disable constraining.
+		 */
+		if (!freq)
+			return 0;
+
 		mclk_freq = clk_get_rate(priv->mclk);
 		/* enable FLL if a different sysclk is desired */
 		if (mclk_freq != freq) {