rapidio/mport_cdev: mark expected switch fall-through

In preparation for enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warning:

  drivers/rapidio/devices/rio_mport_cdev.c: In function `mport_release_mapping':
  drivers/rapidio/devices/rio_mport_cdev.c:2151:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
     rio_unmap_inb_region(mport, map->phys_addr);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    CC      drivers/regulator/fixed-helper.o
    CC      drivers/pinctrl/stm32/pinctrl-stm32f429.o
  drivers/rapidio/devices/rio_mport_cdev.c:2152:2: note: here
    case MAP_DMA:
    ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Link: http://lkml.kernel.org/r/20190212175014.GA14326@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Alexandre Bounine <alex.bou9@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>