mmc: sdhci-xenon: Fixup already marked switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
according to what the compiler looks for, where we are expecting to fall
through.

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