mmc: core: Convert mmc_align_data_size() into an SDIO specific function

The only user of mmc_align_data_size() is sdio_align_size(), which is
called from SDIO func drivers to let them distinguish, how to optimally
allocate data buffers.

Let's move mmc_align_data_size() close to the SDIO code as to make it
static, rename it to _sdio_align_size() and simplify its definition, all
with the purpose of clarifying that this is SDIO specific.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Avri Altman <avri.altman@wdc.com>