dma: mxs-dma: make mxs_dma_prep_slave_sg() multi user safe

Using a static variable for counting the number of CCWs attached to
a DMA channel when appending a new descriptor is not multi user safe.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>