mmc: alcor: enable DMA for writes

Enable the DMA codepath for writes as well as reads.
This improves write speed from 1mb/sec to 2mb/sec (tested with dd).

The original ampe_stor vendor driver also uses DMA for writes.

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