mmc: sdhci-s3c: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/mmc/host/sdhci-s3c.c: In function 'sdhci_s3c_probe':
drivers/mmc/host/sdhci-s3c.c:613:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
   host->mmc->caps |= MMC_CAP_8_BIT_DATA;
drivers/mmc/host/sdhci-s3c.c:614:2: note: here
  case 4:
  ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>