mmc: meson-mx-sdio: mark expected switch fall-through

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

Notice that in this particular case, I replaced the "fall-through:"
comment with a proper "fall through", which is what GCC is
expecting to find.

Addresses-Coverity-ID: 1373880 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>