ASoC: intel: atom: localize variable without external linkage

A driver for Intel SST driver for old atom platform includes a variable
which has no external linkage. These functions should have static
qualifier.

This commit adds the qualifier to localize the variable. This issue is
detected by sparse:

sst.c:261:1: warning: symbol 'dev_attr_firmware_version' was not declared. Should it be static?

Cc: Sebastien Guiriec <sebastien.guiriec@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/intel/atom/sst/sst.c

@@ -258,7 +258,7 @@ static ssize_t firmware_version_show(struct device *dev,
 
 }
 
-DEVICE_ATTR_RO(firmware_version);
+static DEVICE_ATTR_RO(firmware_version);
 
 static const struct attribute *sst_fw_version_attrs[] = {
 	&dev_attr_firmware_version.attr,