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.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/serio/libps2.c

@@ -405,7 +405,7 @@ bool ps2_handle_ack(struct ps2dev *ps2dev, u8 data)
 			ps2dev->nak = PS2_RET_ERR;
 			break;
 		}
-		/* Fall through */
+		fallthrough;
 
 	/*
 	 * Workaround for mice which don't ACK the Get ID command.