watchdog: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword instead of letting the
code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>