mmc: core: let the dma map ops handle bouncing

mmc: core: let the dma map ops handle bouncing

Just like we do for all other block drivers. Especially as the limit
imposed at the moment might be way to pessimistic for iommus.

This also means we are not going to set a bounce limit for the queue, in
case we have a dma mask. On most architectures it was never needed, the
major hold out was x86-32 with PAE, but that has been fixed by now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>