hw/dma/omap_dma: Move switch 'fall through' comment to correct place

hw/dma/omap_dma: Move switch 'fall through' comment to correct place

Reported by GCC9 when building with CFLAG -Wimplicit-fallthrough=2:

    CC      hw/dma/omap_dma.o
  hw/dma/omap_dma.c: In function ‘omap_dma_write’:
  hw/dma/omap_dma.c:1532:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
   1532 |         if (s->model <= omap_dma_3_1)
        |            ^
  hw/dma/omap_dma.c:1534:5: note: here
   1534 |     case 0x400:
        |     ^~~~
  cc1: all warnings being treated as errors

Correctly place the 'fall through' comment.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20190719131425.10835-3-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>