ASoC: sgtl5000: defer the probe if clock is not found

It's not always the case that clock is already available when sgtl5000
get probed at the first time, e.g. the clock is provided by CPU DAI
which may be probed after sgtl5000.  So let's defer the probe when
devm_clk_get() call fails and give it chance to try later.

It fixes the regression on imx28 since commit 9e13f34 (ASoC: sgtl5000:
Let the codec acquire its clock).

[    1.927637] sgtl5000 0-000a: Failed to get mclock: -2
[    1.934280] sgtl5000: probe of 0-000a failed with error -2
[    1.945906] mxs-sgtl5000 sound.13: ASoC: CODEC (null) not registered
[    1.953787] mxs-sgtl5000 sound.13: snd_soc_register_card failed (-517)
[    1.960865] platform sound.13: Driver mxs-sgtl5000 requests probe deferral

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/codecs/sgtl5000.c

@@ -1527,6 +1527,9 @@ static int sgtl5000_i2c_probe(struct i2c_client *client,
 	if (IS_ERR(sgtl5000->mclk)) {
 		ret = PTR_ERR(sgtl5000->mclk);
 		dev_err(&client->dev, "Failed to get mclock: %d\n", ret);
+		/* Defer the probe to see if the clk will be provided later */
+		if (ret == -ENOENT)
+			return -EPROBE_DEFER;
 		return ret;
 	}