mmc: core: annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warning (W=1).

In this particular case rewrote the comment to start with the string "fall
through", so as to match the regular expression expected by GCC. Truncate
the comment slightly to fit the max line length of 80 characters.

This commit remove the following warning:

  drivers/mmc/core/host.c:196:14: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>