media: stm32: dcmipp: correct ret type in dcmipp_graph_notify_bound

The ret variable used within the function dcmipp_graph_notify_bound is
wrongly defined as unsigned int while it can also be signed.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>