mmc: sh_mmcif: remove unneeded struct sh_mmcif_dma, prepare to shdma conversion

Now that all users have been updated to use the embedded in struct
sh_mmcif_plat_data DMA slave IDs, struct sh_mmcif_dma is no longer needed
and can be removed. This also makes preparation to the shdma base library
conversion easier.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Chris Ball <cjb@laptop.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>