mmc: sdhci: put the aligned buffer pointer to struct sdhci_host

Using the global variable does not look nice.

Add a new field sthci::align_buffer to point to the bounce buffer.

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