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

Hi Mark,

thanks your insisting on a better description for the patch, I found a
more appropriate solution for the problem:

Compiling the SoC Audio driver for Freescale i.MX as a module
(CONFIG_SND_SOC_IMX_PCM=m) results in a non-functional sound driver
indicated by the error message:
| 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
instead of the message:
| imx-sgtl5000 sound.1:  sgtl5000 <-> 63fcc000.ssi mapping ok
that is to be expected upon loading the snd-soc-imx-pcm.ko module.

The build log reveals, that the file imx-pcm-dma.o (or imx-pcm-fiq.o
depending on the kernel configuration), which should be linked
together with imx-pcm.o into snd-imx-pcm.ko, is not being compiled in
this case.

The make rules for these files shows that the target object imx-pcm.o
is assigned to the variable snd-soc-imx-pcm-y while
imx-pcm-{dma,fiq}.o are added to to
snd-soc-imx-pcm-$(CONFIG_SND_SOC_IMX_PCM_DMA) and
snd-soc-imx-pcm-$(CONFIG_SND_SOC_IMX_PCM_FIQ) which resolve to
snd-soc-imx-pcm-m in this case.

According to Documentation/kbuild/modules.txt:
|When the module is built from multiple sources, an additional line is
|needed listing the files:
|
|        <module_name>-y := <src1>.o <src2>.o ...
Thus the type of the config variables CONFIG_SND_SOC_IMX_PCM_DMA and
CONFIG_SND_SOC_IMX_PCM_FIQ should be 'bool' instead of 'tristate' to
resolve to 'y' when selected.

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