dmaengine: fsldma: Mark expected switch fall-through

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 <[e-mail]>
Acked-by: Li Yang <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

drivers/dma/fsldma.c

···
 	switch (chan->feature & FSL_DMA_IP_MASK) {
 	case FSL_DMA_IP_85XX:
 		chan->toggle_ext_pause = fsl_chan_toggle_ext_pause;
+		/* Fall through */
 	case FSL_DMA_IP_83XX:
 		chan->toggle_ext_start = fsl_chan_toggle_ext_start;
 		chan->set_src_loop_size = fsl_chan_set_src_loop_size;