[media] v4l: omap4iss: Don't use v4l2_g_ext_ctrls() internally

Instead of using the extended control API internally to get the sensor
pixel rate, use the dedicated in-kernel APIs (find the control with
v4l2_ctrl_find() and get its value with v4l2_ctrl_g_ctrl_int64()).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>