mmc: tifm_sd: 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
"deliberate fall-through" comment with a proper "fall through"
at the bottom of the case, which is what GCC is expecting to find.

Addresses-Coverity-ID: 1373887 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>