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>