spapr_events: Rewrite a fall through comment

spapr_events: Rewrite a fall through comment

GCC9 is confused by this comment when building with CFLAG
-Wimplicit-fallthrough=2:

    CC      ppc64-softmmu/hw/ppc/spapr_rtc.o
  hw/ppc/spapr_events.c: In function ‘rtas_event_log_to_source’:
  hw/ppc/spapr_events.c:312:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
    312 |         if (spapr_ovec_test(spapr->ov5_cas, OV5_HP_EVT)) {
        |            ^
  hw/ppc/spapr_events.c:317:5: note: here
    317 |     case RTAS_LOG_TYPE_EPOW:
        |     ^~~~
  cc1: all warnings being treated as errors

Rewrite the comment using 'fall through' which is recognized by
GCC and static analyzers.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190719131425.10835-8-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>