mmc: core: Remove unused struct _mmc_csd from public mmc.h header

The struct _mmc_csd isn't being used and has been lurking around for a
while. Let's kill it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>