media: stm32-dcmi: fix check of pm_runtime_get_sync return value

Start streaming was sometimes failing because of pm_runtime_get_sync()
non-0 return value. In fact return value was not an error but a
positive value (1), indicating that PM was already enabled.
Fix this by going to error path only with negative return value.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>