hw/omap_dma, hw/omap_spi: Explicitly mark fallthroughs

hw/omap_dma, hw/omap_spi: 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 <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>