ASoC: dwc: Fix -Wmissing-prototypes warnings

while CONFIG_SND_DESIGNWARE_PCM is not set, building with W=1 shows this:

sound/soc/dwc/local.h:127:6: warning: no previous prototype for ‘dw_pcm_push_tx’ [-Wmissing-prototypes]
 void dw_pcm_push_tx(struct dw_i2s_dev *dev) { }
      ^~~~~~~~~~~~~~
sound/soc/dwc/local.h:128:6: warning: no previous prototype for ‘dw_pcm_pop_rx’ [-Wmissing-prototypes]
 void dw_pcm_pop_rx(struct dw_i2s_dev *dev) { }
      ^~~~~~~~~~~~~
sound/soc/dwc/local.h:129:5: warning: no previous prototype for ‘dw_pcm_register’ [-Wmissing-prototypes]
 int dw_pcm_register(struct platform_device *pdev)
     ^~~~~~~~~~~~~~~

Change these to inline functions to fix the warnings.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20210329150524.18184-1-yuehaibing@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/dwc/local.h

@@ -124,9 +124,9 @@ void dw_pcm_push_tx(struct dw_i2s_dev *dev);
 void dw_pcm_pop_rx(struct dw_i2s_dev *dev);
 int dw_pcm_register(struct platform_device *pdev);
 #else
-void dw_pcm_push_tx(struct dw_i2s_dev *dev) { }
-void dw_pcm_pop_rx(struct dw_i2s_dev *dev) { }
-int dw_pcm_register(struct platform_device *pdev)
+static inline void dw_pcm_push_tx(struct dw_i2s_dev *dev) { }
+static inline void dw_pcm_pop_rx(struct dw_i2s_dev *dev) { }
+static inline int dw_pcm_register(struct platform_device *pdev)
 {
 	return -EINVAL;
 }