ce4100: fix a build error

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: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>