mmc: pxamci: remove the dmaengine compat need

As the pxa architecture switched towards the dmaengine slave map, the
old compatibility mechanism to acquire the dma requestor line number and
priority are not needed anymore.

This patch simplifies the dma resource acquisition, using the more
generic function dma_request_slave_channel().

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>