dmaengine: increment privatecnt when using dma_get_any_slave_channel

Channels allocated via dma_get_any_slave_channel were not increasing
the counter tracking private allocations.  When these channels were
released, privatecnt may erroneously fall to zero.  The DMA device
would then lose its DMA_PRIVATE cap and fail to allocate future private
channels (via private_candidate) as any allocations still outstanding
would incorrectly be seen as public allocations.

Signed-off-by: Christopher Freeman <cfreeman@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>