mmc: queue: use bigger segments if DMA MAP layer can merge the segments

When the max_segs of a mmc host is smaller than 512, the mmc
subsystem tries to use 512 segments if DMA MAP layer can merge
the segments, and then the mmc subsystem exposes such information
to the block layer by using blk_queue_can_use_dma_map_merging().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>