media: mtk-vcodec: venc: fix return value when start_streaming fails

In case vb2ops_venc_start_streaming fails, the error value
is overwritten by the ret value of pm_runtime_put which might
be 0. Fix it.

Fixes: 985c7369 (" media: mtk-vcodec: Separating mtk encoder driver")
Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>