Input: libps2 - fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by replacing a /* Fall through */ comment with the new
pseudo-keyword macro fallthrough.

Notice that Clang doesn't recognize /* Fall through */ comments as
implicit fall-through markings.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/d2944854e3e118b837755abf4cbdb497662001b7.1605896060.git.gustavoars@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>