drivers: video: sw_generator: Fix stream_start return value

The stream_start() callback needs to return 0 for success. However, to
indicate work has been successfully scheduled, k_work_schedule() could
return either 0 (already scheduled) or 1 (newly scheduled).

Due to this issue, the sw_generator could not start streaming. Fix it.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>