Revert "mmc: alcor: enable DMA transfer of large buffers"

This reverts commit 57ebb96c293da9f0ec56aba13c5541269a5c10b1.

Usage of the DMA page iterator was problematic here because
we were not considering offset & length of entries in the scatterlist.

Also, after further discussion, the suggested revised approach is much
more similar to the driver implementation before this commit was
applied, so revert it.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>