media: rockchip/rga: Fix broken .start_streaming

Currently, rga_buf_start_streaming() is expecting
pm_runtime_get_sync to return zero on success, which
is wrong.

As per the documentation, pm_runtime_get_sync increments
the device's usage counter and return its result.
This means it will typically return a positive integer
on success and a negative error code.

Therefore, rockchip-rga driver is currently unusable
failing to start_streaming in most cases. Fix it and
while here, cleanup the buffer return-to-core logic.

Fixes: f7e7b48e ("[media] rockchip/rga: v4l2 m2m support")

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Jacob Chen <jacob-chen@iotwrt.com>
[hans.verkuil@cisco.com: fix line over 80 cols warning]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>