ray_cs: Use fallthrough pseudo-keyword

In order to enable -Wimplicit-fallthrough for Clang[1], replace the
existing /* fall through */ comments with the new pseudo-keyword
macro fallthrough[2].

[1] https://git.kernel.org/linus/e2079e93f562c7f7a030eb7642017ee5eabaaa10
[2] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/20201008220422.GA6926@embeddedor

drivers/net/wireless/ray_cs.c

···
 	switch (ccsindex = get_free_tx_ccs(local)) {
 	case ECCSBUSY:
 		pr_debug("ray_hw_xmit tx_ccs table busy\n");
-		/* fall through */
+		fallthrough;
 	case ECCSFULL:
 		pr_debug("ray_hw_xmit No free tx ccs\n");
-		/* fall through */
+		fallthrough;
 	case ECARDGONE:
 		netif_stop_queue(dev);
 		return XMIT_NO_CCS;
···
 	switch (wrqu->mode) {
 	case IW_MODE_ADHOC:
 		card_mode = 0;
-		/* Fall through */
+		fallthrough;
 	case IW_MODE_INFRA:
 		local->sparm.b5.a_network_type = card_mode;
 		break;