ENGR00174395 V4L2 capture: Driver improvement

ENGR00174395 V4L2 capture: Driver improvement

1) CSI module should be disabled first for CSI_MEM channel,
otherwise, the capture channels will hang after restarting
for several times.
2) Disable CSI module correctly for overlay. Move stopping
preview channel operation out of de-select interface.
3) Check cam->overlay_on is true in close function before
stopping preview.
4) Check cam->vf_start_sdc function before calling it.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>