mmc: jz4740: Annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1).

This commit removes the following warnings:

  drivers/mmc/host/jz4740_mmc.c:745:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/mmc/host/jz4740_mmc.c:779:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>