Input: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a fallthrough; statement.

Notice that this seems to be a Duff device for performance[1]. So,
although the code looks a bit _funny_, I didn't want to refactor
or modify it beyond merely adding a fallthrough marking, which
might be the least disruptive way to fix this issue.

[1] https://www.drdobbs.com/a-reusable-duff-device/184406208

Link: KSPP#115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Author: GustavoARSilva

drivers/input/joystick/sidewinder.c

@@ -660,6 +660,7 @@ static int sw_connect(struct gameport *gameport, struct gameport_driver *drv)
 					fallthrough;
 				case 45:				/* Ambiguous packet length */
 					if (j <= 40) {			/* ID length less or eq 40 -> FSP */
+					fallthrough;
 				case 43:
 						sw->type = SW_ID_FSP;
 						break;