media: ov5640: fix framerate update

After a framerate update through s_frame_interval(), the new
framerate was not taken into account when streaming,
but was taken into account on next session.
This was due to sensor->current_mode not updated accordingly to new
framerate setting in ov5640_s_frame_interval().

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>