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>