media: stm32: csi: correct unsigned or useless variable settings

Correct several missing unsigned type missing for loop variables
and also remove useless initialization of variables.

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>