dmaengine: dw: lazy allocation of dma descriptors

This patch changes the driver to allocate DMA descriptors when
needed. This stops memory resources to be wasted and letting
them sit idle in the free_list structure when the device doesn't
need it... This also solves the problem, that a driver has to
guess the number of how many descriptors it needs to allocate
in advance. Currently, the dma engine will just fail when put
under load by sata_dwc_460ex.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>