ASoC: fsl-asoc-card: move 'config SND_SOC_FSL_ASOC_CARD' to 'if SND_IMX_SOC'

ASoC: fsl-asoc-card: move 'config SND_SOC_FSL_ASOC_CARD' to 'if SND_IMX_SOC'

Build kernel with SND_SOC_FSL_ASOC_CARD=m && SND_SOC_FSL_{SSI,SAI,ESAI}=y
leads the following error:

   sound/built-in.o: In function `fsl_sai_probe':
>> fsl_sai.c:(.text+0x5f662): undefined reference to `imx_pcm_dma_init'
   sound/built-in.o: In function `fsl_esai_probe':
>> fsl_esai.c:(.text+0x6044b): undefined reference to `imx_pcm_dma_init'

The config SND_SOC_FSL_ASOC_CARD is for IMX SOC, So move it under condition
of 'if SND_IMX_SOC'.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>