Input: elantech - annotate fall-through case in elantech_use_host_notify()

This avoids a warning when building with -Wimplicit-fallthrough.

Fixes: 883a2a80 ("Input: elantech - enable SMBus on new (2018+) systems")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/elantech.c

@@ -1842,6 +1842,7 @@ static bool elantech_use_host_notify(struct psmouse *psmouse,
 		/* SMbus implementation is stable since 2018 */
 		if (dmi_get_bios_year() >= 2018)
 			return true;
+		/* fall through */
 	default:
 		psmouse_dbg(psmouse,
 			    "Ignoring SMBus bus provider %d\n", info->bus);