media: stm32: csi: remove useless fwnode_graph_get_endpoint call

The endpoint is already retrieved at the beginning of the function
stm32_csi_parse_dt hence keep the endpoint pointer until the end
instead of getting a new one.

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>