dmaengine: jz4780: Use dma_get_slave_channel when requesting a specific channel

When the DT requests a specific channel to use it is not necesssary
to scan through all DMA channels in the system. Just return the
requested channel using dma_get_slave_channel().

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>