hw/pxa2xx_timer: Explicitly mark fallthroughs

hw/pxa2xx_timer: Explicitly mark fallthroughs

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: Peter Maydell <[email protected]>
Signed-off-by: Blue Swirl <[email protected]>