ASoC: SOF: SND_INTEL_DSP_CONFIG dependency

ASoC: SOF: SND_INTEL_DSP_CONFIG dependency

The sof-pci-dev driver fails to link when built into the kernel
and CONFIG_SND_INTEL_DSP_CONFIG is set to =m:

arm-linux-gnueabi-ld: sound/soc/sof/sof-pci-dev.o: in function `sof_pci_probe':
sof-pci-dev.c:(.text+0x1c): undefined reference to `snd_intel_dsp_driver_probe'

As a temporary fix, use IS_REACHABLE to prevent the problem from
happening. A more complete solution is to move this code to
Intel-specific parts, restructure the drivers and Kconfig as discussed
with Arnd Bergmann and Takashi Iwai.

Fixes: 82d9d54a6c0e ("ALSA: hda: add Intel DSP configuration / probe code")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210122005725.94163-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>