libpayload/drivers/i8042: Add fallthrough comment

Ctrl-delete does nothing, so it falls through to the default case.
Add a comment to make this explicit.

Found-by: Coverity Scan #1260878
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Change-Id: I4a6f51cb04696b6ebcb554c5667a5bbea58622c1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32750
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

payloads/libpayload/drivers/i8042/keyboard.c

@@ -258,6 +258,7 @@ int keyboard_getchar(void)
 				/* vulcan nerve pinch */
 				if ((modifier & KB_MOD_ALT) && reset_handler)
 					reset_handler();
+				/* fallthrough */
 			default:
 				ret = 0;
 			}