DMA: Freescale: add fsl_dma_free_descriptor() to reduce code duplication

There are several places where descriptors are freed using identical code.
This patch puts this code into a function to reduce code duplication.

Signed-off-by: Hongbo Zhang <hongbo.zhang@freescale.com>
Signed-off-by: Qiang Liu <qiang.liu@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>