v4l: xilinx-multi-scaler: Allow device run for single channel

v4l: xilinx-multi-scaler: Allow device run for single channel

Start the processing of channels whose stream_on called.
This removes the dependency that scaling starts
only after all channel calls the stream_on.

Signed-off-by: Suresh Gupta <suresh.gupta@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Satish Kumar Nagireddy <satish.nagireddy.nagireddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>