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>

sound/soc/sof/ops.c

@@ -17,7 +17,7 @@ bool snd_sof_pci_update_bits_unlocked(struct snd_sof_dev *sdev, u32 offset,
 {
 	struct pci_dev *pci = to_pci_dev(sdev->dev);
 	unsigned int old, new;
-	u32 ret;
+	u32 ret = 0;
 
 	pci_read_config_dword(pci, offset, &ret);
 	old = ret;