dmaengine: cppi41: init_sched(): Get number of channels from DT

Despite the driver is already using DT to get the number of channels,
init_sched() is using an hardcoded value to get it.
Use DT to get the number of channels.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>