ASoC: SOF: Fix a compile warning with CONFIG_PCI=n

A trivial fix for the randconfig build error:
  sound/soc/sof/ops.c:20:6: warning: ‘ret’ is used uninitialized in this function [-Wuninitialized]

Fixes: d1d95fcb

 ("ASoC: SOF: Add DSP HW abstraction operations")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>