ssb: chipcommon: fix fall-through annotation

Replace "Fallthough" with a proper "fall through" annotation.

This fix is part of the ongoing efforts to enabling
-Wimplicit-fallthrough

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Michael Buesch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>