Mark switch cases where we are expecting to fall through.

Fix the following warnings (Building: powerpc-ppa8548_defconfig powerpc):

drivers/dma/fsldma.c: In function ‘fsl_dma_chan_probe’:
drivers/dma/fsldma.c:1165:26: warning: this statement may fall through [-Wimplicit-fallthrough=]
   chan->toggle_ext_pause = fsl_chan_toggle_ext_pause;
   ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/dma/fsldma.c:1166:2: note: here
  case FSL_DMA_IP_83XX:
  ^~~~
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>