dma: pl330: split off common code to give back descriptors

This patch adds __pl330_giveback_descs which give back descriptors when fails
allocating descriptors. It requires to eliminate duplication for
pl330_prep_dma_sg which will be added later.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Acked-by : Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>