misc: mic/scif: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/misc/mic/scif/scif_rma.c: In function ‘scif_unregister_window’:
drivers/misc/mic/scif/scif_rma.c:665:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
   send_msg = true;
   ~~~~~~~~~^~~~~~
drivers/misc/mic/scif/scif_rma.c:668:2: note: here
  case OP_IN_PROGRESS:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>