mmc: atmel-mci: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/mmc/host/atmel-mci.c: In function 'atmci_get_cap':
drivers/mmc/host/atmel-mci.c:2415:30: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->caps.has_odd_clk_div = 1;
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
drivers/mmc/host/atmel-mci.c:2416:2: note: here
  case 0x400:
  ^~~~
drivers/mmc/host/atmel-mci.c:2422:28: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->caps.has_highspeed = 1;
   ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
drivers/mmc/host/atmel-mci.c:2423:2: note: here
  case 0x200:
  ^~~~
drivers/mmc/host/atmel-mci.c:2426:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->caps.need_notbusy_for_read_ops = 1;
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
drivers/mmc/host/atmel-mci.c:2427:2: note: here
  case 0x100:
  ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>