drm/bridge: sii902x: make audio mclk optional

The master clock on i2s bus is not mandatory,
as sii902X internal PLL can be used instead.
Make use of mclk optional.
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563811560-29589-4-git-send-email-olivier.moysan@st.com

drivers/gpu/drm/bridge/sii902x.c

@@ -568,13 +568,14 @@ static int sii902x_audio_hw_params(struct device *dev, void *data,
 		return ret;
 	}
 
-	mclk_rate = clk_get_rate(sii902x->audio.mclk);
-
-	ret = sii902x_select_mclk_div(&i2s_config_reg, params->sample_rate,
-				      mclk_rate);
-	if (mclk_rate != ret * params->sample_rate)
-		dev_dbg(dev, "Inaccurate reference clock (%ld/%d != %u)\n",
-			mclk_rate, ret, params->sample_rate);
+	if (sii902x->audio.mclk) {
+		mclk_rate = clk_get_rate(sii902x->audio.mclk);
+		ret = sii902x_select_mclk_div(&i2s_config_reg,
+					      params->sample_rate, mclk_rate);
+		if (mclk_rate != ret * params->sample_rate)
+			dev_dbg(dev, "Inaccurate reference clock (%ld/%d != %u)\n",
+				mclk_rate, ret, params->sample_rate);
+	}
 
 	mutex_lock(&sii902x->mutex);
 
@@ -751,11 +752,11 @@ static int sii902x_audio_codec_init(struct sii902x *sii902x,
 		sii902x->audio.i2s_fifo_sequence[i] |= audio_fifo_id[i] |
 			i2s_lane_id[lanes[i]] |	SII902X_TPI_I2S_FIFO_ENABLE;
 
-	sii902x->audio.mclk = devm_clk_get(dev, "mclk");
+	sii902x->audio.mclk = devm_clk_get_optional(dev, "mclk");
 	if (IS_ERR(sii902x->audio.mclk)) {
 		dev_err(dev, "%s: No clock (audio mclk) found: %ld\n",
 			__func__, PTR_ERR(sii902x->audio.mclk));
-		return 0;
+		return PTR_ERR(sii902x->audio.mclk);
 	}
 
 	sii902x->audio.pdev = platform_device_register_data(