dmaengine: fix balance of privatecnt

dma_release_channel() decrements privatecnt counter and almost all dma_get*
function increments it with the exception of dma_get_slave_channel().
In most cases this does not cause issue since normally the channel is not
requested and released, but if a driver requests DMA channel via
dma_get_slave_channel() and releases the channel the privatecnt will be
unbalanced and this will prevent for example getting channel for memcpy.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>