dmaengine: omap-dma: Reduce the number of virtual channels

Since the mapping between the hardware request lines and channels has been
removed it no longer make sense to have too many channels.
Set the number of channels to match with the number of logical channels
supported by sDMA.

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