mmc: sd: Meet alignment requirements for raw_ssr DMA

The mmc_read_ssr() function results in DMA to the raw_ssr member of
struct mmc_card, which is not guaranteed to be cache line aligned & thus
might not meet the requirements set out in Documentation/DMA-API.txt:

  Warnings:  Memory coherency operates at a granularity called the cache
  line width.  In order for memory mapped by this API to operate
  correctly, the mapped region must begin exactly on a cache line
  boundary and end exactly on one (to prevent two separately mapped
  regions from sharing a single cache line).  Since the cache line size
  may not be known at compile time, the API will not enforce this
  requirement.  Therefore, it is recommended that driver writers who
  don't take special care to determine the cache line size at run time
  only map virtual regions that begin and end on page boundaries (which
  are guaranteed also to be cache line boundaries).

On some systems where DMA is non-coherent this can lead to us losing
data that shares cache lines with the raw_ssr array.

Fix this by kmalloc'ing a temporary buffer to perform DMA into. kmalloc
will ensure the buffer is suitably aligned, allowing the DMA to be
performed without any loss of data.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Fixes: 5275a652 ("mmc: sd: Export SD Status via “ssr” device attribute")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>