Explicitly mark the fallthroughs as intentional in the code
pattern where we gradually increment an index before falling
into the code to read/write that array entry:
    case THINGY_3: idx++;
    case THINGY_2: idx++;
    case THINGY_1: idx++;
    case THINGY_0: return s->thingy[idx];

This makes static analysers happy.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>