DMA ENGINE: Do not reset 'private' of channel

The member 'private' of 'struct dma_chan' is meant for passing
data between client and the controller driver.

The DMA client driver may point it to platform specific stuff after
acquiring the channel. So, it is the responsiblity of the same code
to reset it, if it must.

The DMA engine doesn't set it and hence, shouldn't reset it either.

This reseting of private by DMA Engine comes in the way of implementing
default channel settings during DMAC probe. That capability is useful
for not having the clients to always provide platform specific data,
like Rx/Tx FIFO addresses, which usually doesn't change across channel
requests.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>