ASoC: fsl: fix miscompilation of snd-soc-imx-pcm

Due to a broken make rule, sound/soc/fsl/imx-pcm-dma.c or
sound/soc/fsl/imx-pcm-fiq.c (whatever is selected via Kconfig) will
not be compiled into imx-pcm.o when building as module, i.e.:
CONFIG_SND_SOC_IMX_PCM=m
CONFIG_SND_SOC_IMX_PCM_DMA=m
resulting in a non-functional sound driver.

This gives the error messages:
| imx-sgtl5000 sound.1: platform imx-pcm-audio not registered
| imx-sgtl5000 sound.1: snd_soc_register_card failed (-517)
| platform sound.1: Driver imx-sgtl5000 requests probe deferral
when loading the driver instead of what's to be expected:
| imx-sgtl5000 sound.1:  sgtl5000 <-> 63fcc000.ssi mapping ok

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>