video: stm32: dcmi: fix video interface properties read

This commit fixes an error introduced by commit c0dae0c9
("video: stm32: dcmi: perform config based on endpoint properties")
in which incorrect instance was being access leading to usage of
the default value for pixel clock / hsync / vsync polarities and bus
width.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>