ASoC: rt5677: Fix build error without CONFIG_SPI

If CONFIG_SPI is n, SND_SOC_RT5677_SPI also is n, building fails:

sound/soc/codecs/rt5677.o: In function `rt5677_irq':
rt5677.c:(.text+0x2dbf): undefined reference to `rt5677_spi_hotword_detected'
sound/soc/codecs/rt5677.o: In function `rt5677_dsp_work':
rt5677.c:(.text+0x3709): undefined reference to `rt5677_spi_write'

This adds stub helpers to fix this.
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 461c6232 ("ASoC: rt5677: Load firmware via SPI using delayed work")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20191127082145.6100-1-yuehaibing@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt5677-spi.h

@@ -9,9 +9,25 @@
 #ifndef __RT5677_SPI_H__
 #define __RT5677_SPI_H__
 
+#if IS_ENABLED(CONFIG_SND_SOC_RT5677_SPI)
 int rt5677_spi_read(u32 addr, void *rxbuf, size_t len);
 int rt5677_spi_write(u32 addr, const void *txbuf, size_t len);
 int rt5677_spi_write_firmware(u32 addr, const struct firmware *fw);
 void rt5677_spi_hotword_detected(void);
+#else
+static inline int rt5677_spi_read(u32 addr, void *rxbuf, size_t len)
+{
+	return -EINVAL;
+}
+static inline int rt5677_spi_write(u32 addr, const void *txbuf, size_t len)
+{
+	return -EINVAL;
+}
+static inline int rt5677_spi_write_firmware(u32 addr, const struct firmware *fw)
+{
+	return -EINVAL;
+}
+static inline void rt5677_spi_hotword_detected(void){}
+#endif
 
 #endif /* __RT5677_SPI_H__ */