ASoC: intel: atom: localize variable without external linkage

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 <[email protected]>
Cc: Vinod Koul <[email protected]>
Signed-off-by: Takashi Sakamoto <[email protected]>
Signed-off-by: Mark Brown <[email protected]>