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/audio/gusemu_hal.c

@@ -261,6 +261,7 @@ void gus_write(GUSEmuState * state, int port, int size, unsigned int data)
             GUSregb(IRQStatReg2x6) = 0x10;
             GUS_irqrequest(state, state->gusirq, 1);
         }
+        /* fall through */
     case 0x20D:                /* SB2xCd no IRQ */
         GUSregb(SB2xCd) = (uint8_t) data;
         break;