Fix a build error. CE4100 with no serial errors because the alternate
function is only a prototype not a null function as intended.
Signed-off-by: NZhang Rui <rui.zhang@intel.com>
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>