mmc: sdhci: remove unneeded casts

host->mmc is already (struct mmc *).

memalign() returns an opaque pointer, so there is no need for casting.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>