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.com
Signed-off-by: Mark Brown <broonie@kernel.org>