sgi-xpc: 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/sgi-xp/xpc_uv.c: In function ‘xpc_handle_activate_mq_msg_uv’:
drivers/misc/sgi-xp/xpc_uv.c:573:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   xpc_wakeup_channel_mgr(part);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/misc/sgi-xp/xpc_uv.c:575:2: note: here
  case XPC_ACTIVATE_MQ_MSG_MARK_ENGAGED_UV:
  ^~~~

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

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

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Robin <Holt&lt;robinmholt@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>