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

This patch fixes the following warning:

drivers/message/fusion/mptbase.c: In function  mptbase_reply :
drivers/message/fusion/mptbase.c:643:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (event != MPI_EVENT_EVENT_CHANGE)
      ^
drivers/message/fusion/mptbase.c:646:2: note: here
  case MPI_FUNCTION_CONFIG:
  ^~~~

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: Martin K. Petersen <martin.petersen@oracle.com>