scsi: esp: respect FIFO invariant after message phase

The FIFO contains two bytes; hence the write ptr should be two bytes ahead
of the read pointer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit d020aa504c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/scsi/esp.c

@@ -222,7 +222,7 @@ static void write_response(ESPState *s)
     } else {
         s->ti_size = 2;
         s->ti_rptr = 0;
-        s->ti_wptr = 0;
+        s->ti_wptr = 2;
         s->rregs[ESP_RFLAGS] = 2;
     }
     esp_raise_irq(s);