mmc: rtsx_usb_sdmmc: Fix clang -Wimplicit-fallthrough in sd_set_power_mode()

mmc: rtsx_usb_sdmmc: Fix clang -Wimplicit-fallthrough in sd_set_power_mode()

Clang warns (or errors with CONFIG_WERROR=y):

  drivers/mmc/host/rtsx_usb_sdmmc.c:1042:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
   1042 |         case MMC_POWER_UNDEFINED:
        |         ^
  drivers/mmc/host/rtsx_usb_sdmmc.c:1042:2: note: insert 'break;' to avoid fall-through
   1042 |         case MMC_POWER_UNDEFINED:
        |         ^
        |         break;

Clang is a little more pedantic than GCC, which does not warn when
falling through to a case that is just break or return. Clang's version
is more in line with the kernel's own stance in deprecated.rst, which
states that all switch/case blocks must end in either break,
fallthrough, continue, goto, or return. Add the missing break to silence
the warning.

Fixes: 2c3809bd6f65 ("mmc: rtsx_usb_sdmmc: Add 74 clocks in poweron flow")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250620-mmc-rtsx-usb-sdmmc-fix-clang-implicit-fallthrough-v1-1-4031d11159c0@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>