MLK-10115: ov5640:define function ov5640_turn_on_AE_AG() with static

ov5640 driver will failed to build with build-in:
drivers/media/platform/mxc/subdev/built-in.o: In function
`ov5640_turn_on_AE_AG':
ov5640.c:(.text+0x3890): multiple definition of `ov5640_turn_on_AE_AG'
drivers/media/platform/mxc/capture/built-in.o:v4l2-int-device.c:(.text+0x783c):
first defined here
make[3]: *** [drivers/media/platform/built-in.o] Error 1
make[2]: *** [drivers/media/platform] Error 2
make[1]: *** [drivers/media] Error 2
make: *** [drivers] Error 2
make: *** Waiting for unfinished jobs....

It is caused by function of ov5640_turn_on_AE_AG define as global function,
change it to static function to resolv the issue.

Signed-off-by: Sandor Yu <R01008@freescale.com>