fix "Missing break in switch" coverity reports

Many of these are marked as "intentional/fix required" because they
just need adding a fall through comment.  This is exactly what this
patch does, except for target/mips/translate.c where it is easier to
duplicate the code, and hw/audio/sb16.c where I consulted the DOSBox
sources and decide to just remove the LOG_UNIMP before the fallthrough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/timer/sh_timer.c

@@ -74,6 +74,7 @@ static uint32_t sh_timer_read(void *opaque, hwaddr offset)
     case OFFSET_TCPR:
         if (s->feat & TIMER_FEAT_CAPT)
             return s->tcpr;
+        /* fall through */
     default:
         hw_error("sh_timer_read: Bad offset %x\n", (int)offset);
         return 0;