zd1201: 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; instead of letting the code fall
through to the next case.

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

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/52931e343af25f6dab4bd1d604889d2594a861dd.1605896060.git.gustavoars@kernel.org

drivers/net/wireless/zydas/zd1201.c

@@ -966,7 +966,7 @@ static int zd1201_set_mode(struct net_device *dev,
 			 */
 			zd1201_join(zd, "\0-*#\0", 5);
 			/* Put port in pIBSS */
-			/* Fall through */
+			fallthrough;
 		case 8: /* No pseudo-IBSS in wireless extensions (yet) */
 			porttype = ZD1201_PORTTYPE_PSEUDOIBSS;
 			break;