net: spider_net: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/net/ethernet/toshiba/spider_net.c: In function 'spider_net_release_tx_chain':
drivers/net/ethernet/toshiba/spider_net.c:783:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (!brutal) {
       ^
drivers/net/ethernet/toshiba/spider_net.c:792:3: note: here
   case SPIDER_NET_DESCR_RESPONSE_ERROR:
   ^~~~

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/toshiba/spider_net.c

@@ -788,6 +788,7 @@ spider_net_release_tx_chain(struct spider_net_card *card, int brutal)
 			/* fallthrough, if we release the descriptors
 			 * brutally (then we don't care about
 			 * SPIDER_NET_DESCR_CARDOWNED) */
+			/* Fall through */
 
 		case SPIDER_NET_DESCR_RESPONSE_ERROR:
 		case SPIDER_NET_DESCR_PROTECTION_ERROR: