ASoC: SOF: imx: fix reverse CONFIG_SND_SOC_SOF_OF dependency

updated solution to the problem reported with randconfig:

CONFIG_SND_SOC_SOF_IMX depends on CONFIG_SND_SOC_SOF, but is in
turn referenced by the sof-of-dev driver. This creates a reverse
dependency that manifests in a link error when CONFIG_SND_SOC_SOF_OF
is built-in but CONFIG_SND_SOC_SOF_IMX=m:

sound/soc/sof/sof-of-dev.o:(.data+0x118): undefined reference to `sof_imx8_ops'

use def_trisate to propagate the right settings without select.

Fixes: f4df4e4042 ("ASoC: SOF: imx8: Fix COMPILE_TEST error")
Fixes: 202acc565a ("ASoC: SOF: imx: Add i.MX8 HW support")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101173045.27099-6-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/imx/Kconfig

@@ -11,8 +11,8 @@ config SND_SOC_SOF_IMX_TOPLEVEL
 
 if SND_SOC_SOF_IMX_TOPLEVEL
 
-config SND_SOC_SOF_IMX8
-	tristate "SOF support for i.MX8"
+config SND_SOC_SOF_IMX8_SUPPORT
+	bool "SOF support for i.MX8"
 	depends on IMX_SCU
 	depends on IMX_DSP
 	help
@@ -20,4 +20,8 @@ config SND_SOC_SOF_IMX8
           Say Y if you have such a device.
           If unsure select "N".
 
+config SND_SOC_SOF_IMX8
+	def_tristate SND_SOC_SOF_OF
+	depends on SND_SOC_SOF_IMX8_SUPPORT
+
 endif ## SND_SOC_SOF_IMX_IMX_TOPLEVEL