Since the driver supports only contiguous buffers, there is no need to
manually construct a scatterlist with just a single entry, when there is
a dedicated helper for this purpose.

This patch modifies prepare_dma() function to use available helper instead
of manually creating a scatterlist.
Signed-off-by: NTomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: NMark Brown <broonie@linaro.org>