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

This patch fixes the following warnings:

In file included from drivers/mmc/host/usdhi6rol0.c:9:
drivers/mmc/host/usdhi6rol0.c: In function ‘usdhi6_timeout_work’:
./include/linux/device.h:1483:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  _dev_err(dev, dev_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mmc/host/usdhi6rol0.c:1689:3: note: in expansion of macro ‘dev_err’
   dev_err(mmc_dev(host->mmc), "Invalid state %u\n", host->wait);
   ^~~~~~~
drivers/mmc/host/usdhi6rol0.c:1691:2: note: here
  case USDHI6_WAIT_FOR_CMD:
  ^~~~
drivers/mmc/host/usdhi6rol0.c:1711:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   usdhi6_sg_unmap(host, true);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mmc/host/usdhi6rol0.c:1716:2: note: here
  case USDHI6_WAIT_FOR_DATA_END:
  ^~~~
  CC [M]  drivers/isdn/hisax/hisax_isac.o
drivers/mmc/host/usdhi6rol0.c: In function ‘usdhi6_stop_cmd’:
drivers/mmc/host/usdhi6rol0.c:1338:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (mrq->stop->opcode == MMC_STOP_TRANSMISSION) {
      ^
drivers/mmc/host/usdhi6rol0.c:1343:2: note: here
  default:
  ^~~~~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>