Don't rely on host.h to include the mmc.h header, but instead include it
explicitly because the driver depends on it.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NAdrian Hunter <adrian.hunter@intel.com>