ipmi: kcs_bmc: mark expected switch fall-through in kcs_bmc_handle_data

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

Addresses-Coverity-ID: 1465255 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Cc: Haiyue Wang <haiyue.wang@linux.intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>