Mark switch cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/net/ethernet/ibm/ehea/ehea_main.c: In function 'ehea_mem_notifier':
include/linux/printk.h:311:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/ibm/ehea/ehea_main.c:3253:3: note: in expansion of macro 'pr_info'
   pr_info("memory offlining canceled");
   ^~~~~~~
drivers/net/ethernet/ibm/ehea/ehea_main.c:3256:2: note: here
  case MEM_ONLINE:
  ^~~~

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>