ppc: Add a missing break for PPC6xx_INPUT_TBEN

ppc: Add a missing break for PPC6xx_INPUT_TBEN

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
hw/ppc/ppc.c: In function ‘ppc6xx_set_irq’:
hw/ppc/ppc.c:118:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
  118 |             if (level) {
      |                ^
hw/ppc/ppc.c:123:9: note: here
  123 |         case PPC6xx_INPUT_INT:
      |         ^~~~

According to the discussion, a break statement needs to be added here.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20201116024810.2415819-7-kuhn.chenqun@huawei.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>